Nickel powders, methods for producing powders and devices fabricated from same

ABSTRACT

Nickel powder batches and methods for producing nickel powder batches. The powder batches include particles having a small particle size, narrow size distribution and a spherical morphology. The present invention is also directed to devices incorporating the nickel metal powders.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.09/028,678, filed Feb. 24, 1998, now U.S. Pat. No. 6,316,100, whichclaims priority to U.S. Provisional Patent Application Nos. 60/038,258and 60/039,450, both filed Feb. 24, 1997. Each of the foregoing areincorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH/DEVELOPMENT

This invention was made with Government support under contractsN00014-95-C-0278 and N00014-96-C-0395 awarded by the Office of NavalResearch. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nickel powders and to methods forproducing such powders, as well as intermediate products and devicesfabricated using the powders. In particular, the present invention isdirected to powder batches of nickel particles with a small averageparticle size, well controlled particle size distribution, sphericalmorphology and high crystallinity.

2. Description of Related Art

Many product applications require metal-containing powders with one ormore of the following properties: high purity; high crystallinity; smallaverage particle size; narrow particle size distribution; sphericalparticle morphology; controlled surface chemistry; reduced agglomerationof particles; and high density (low porosity). Examples of metal powdersrequiring such characteristics include, but are not limited to, thoseuseful in microelectronic applications, such as for multilayer ceramiccapacitors (MLCC's), multichip modules and other devices, including flatpanel displays.

Electronic devices such as capacitors, and in particular MLCC's, havetraditionally incorporated electrodes fabricated from noble metals suchas palladium. MLCC's are fabricated by stacking alternate layers of aceramic dielectric and a conductive metal and then sintering (heating)the stack to densify the layers and obtain a monolithic device. Mostceramic dielectric compounds are oxides that must be sintered at anelevated temperature in an oxygen-containing atmosphere to avoidreduction of the ceramic and the loss of the dielectric properties.Noble metals such as palladium advantageously resist oxidation underthese conditions. However, noble metals are relatively expensive andsignificantly increase the fabrication cost of such devices. Therefore,it would be advantageous to utilize less costly base metals for suchapplications. Base metals such as nickel are generally at least an orderof magnitude less costly than noble metals. But most base metals have atendency to oxidize when held in an oxygen-containing atmosphere atelevated sintering temperatures, thereby ruining the electricalproperties of the metal and creating other problems in the device, suchas delamination of the stacked layers.

There have been attempts in the art to address some of the problemsassociated with using base metals in such microelectronic devices. U.S.Pat. No. 3,902,102 by Burn discloses a ceramic capacitor utilizingelectrodes fabricated from nickel or copper powder having a particlesize of less than about 325 mesh (44 μm). The metal is protected fromoxidation during sintering of the capacitor by the addition of a bariumborate glass to the thick film paste composition used to apply theelectrode.

U.S. Pat. Nos. 3,966,463, 4,010,025 and 4,036,634, all by Fraioli etal., disclose an oxidation resistant powder which includes gold ornickel metal and small amounts of a co-nucleated oxide, such as titaniaor zirconia, formed by co-nucleation and precipitation from anammoniacal solution with sodium bisulfite. The powder has good tapdensity, which improves the rheological properties of pastes made fromthe powder. It is disclosed that nickel oxidizes to nickel oxide in airbetween 350° C. and 700° C. and that a high surface area nickel powdercan oxidize at room temperature. The nickel/zirconia powder with about 2weight percent zirconia is able to withstand one hour in air at 450° C.with no measurable weight gain due to oxidation.

U.S. Pat. No. 4,115,493 by Sakabe et al. discloses a ceramic dielectricfor an MLCC that can be sintered in a reducing atmosphere, and thereforepermits nickel electrodes to be utilized. It is disclosed that a paste,including nickel powder having an average particle size of about 1 μm,can be screened onto the ceramic dielectric to form the MLCC structure.

U.S. Pat. No. 4,122,232 by Kuo discloses a thick film paste for forminga conductor, including 50 to 80 weight percent nickel powder and 5 to 20weight percent boron powder. It is disclosed that the boron powderadvantageously reduces oxidation of the nickel powder. In one example,nickel powder having a particle size between about 2.9 μm and 3.6 μm isutilized in the thick film paste.

U.S. Pat. No. 4,223,369 by Burn discloses a zirconate dielectriccomposition including boron that can be sintered in a reducingatmosphere. The use of a reducing atmosphere during sintering allowsnickel electrodes to be used. U.S. Pat. No. 4,700,264 by Kishi et al.also discloses a dielectric that can be sintered in a reducingatmosphere. Nickel electrodes are utilized in the ceramic capacitor andit is disclosed that the nickel powder has an average particle size ofabout 1.5 μm.

U.S. Pat. No. 4,954,926 by Pepin discloses a thick film pastecomposition including organometallics that advantageously reducedelamination defects in the MLCC. The metal powder can include nickelpowder.

It can be seen from the foregoing that there are significant advantagesto using base metals, such as nickel, for the formation of electrodes inmicroelectronic applications or other devices such as flat paneldisplays. Nickel powders are less expensive than noble metals andprovide good conductivity. Nickel metal also resists leaching(degradation) during soldering.

Other uses for fine nickel metal powders include their use to formdispersion strengthened alloys or for porous barriers for the gaseousphase separation of uranium isotopes. Such applications are disclosed inU.S. Pat. No. 3,748,118 by Montino et al. U.S. Pat. No. 3,850,612 byMontino et al. also discloses that spherical nickel metal powders can beadvantageously utilized for slip casting of metal components becausesuch powders yield green castings of greater uniformity and density. Thepowders are also useful as catalysts where the high surface areaaccelerates chemical reactions. The powders can also be used tofabricate porous electrodes and filters or membranes having controlledpermeability. U.S. Pat. No. 4,578,114 by Rangaswamy et al. alsodiscloses the use of composite nickel powders as a thermal spray powderfor the deposition of a thermal spray coating onto a substrate. Asimilar application is also disclosed in U.S. Pat. No. 5,063,021 byAnand et al. Each of these U.S. patents is incorporated herein byreference in its entirety.

Different methods have been proposed to produce nickel metal powders.U.S. Pat. No. 3,711,274 by Montino et al. discloses a process forpreparing spherical, sub-micron nickel powder by heating a suspension ofbis-acrylonitrile-nickel in methanol to produce nickel particles. It isdisclosed that the nickel retains up to 15 percent organic impuritiesand can be purified by hydrogenating the powder at elevatedtemperatures. The average particle size is about 57 nanometers.

U.S. Pat. No. 3,748,118 by Montino et al. discloses a process forproducing spherical nickel powder by heating a hydroalcoholic suspensionof a nickel compound under hydrogen pressure. The average particle sizeof the nickel is from about 0.07 μm to about 2 μm. U.S. Pat. No.3,850,612 by Montino et al. discloses a similar process for producingnickel powder having an average particle size of from about 0.03 μm toabout 0.7 μm.

The article entitled “Preparing Monodispersed Metal Powders inMicrometer and Submicrometer Sizes by the Polyol Process” by Fievet etal. (MRS Bulletin, December, 1989) discloses the preparation of nickelmetal powders by the reduction of nickel hydroxide in ethylene glycol.It is disclosed that the nickel powders have a small size and a narrowsize distribution. As with most liquid preparation routes, the particleshave low crystallinity (i.e. a small average crystallite size). Viau etal., in an article entitled “Preparation and Microwave Characterizationof Spherical and Monodisperse Co₂₀Ni₈₀ Particles” (J. Appl. Phys., 76,(10), 1994), disclose cobalt-nickel alloy particles produced using asimilar process.

Spray pyrolysis is not in common use for the production of nickelpowders containing small particles, such as those having an averageparticle size of not greater than about 5 μm. This is believed to be dueto the high processing costs and low production rates typicallyassociated with spray pyrolysis. Further, spray pyrolysis methods oftenproduce hollow particles that are not sufficiently densified for mostapplications. Generally, spray pyrolysis methods include the generationof liquid droplets wherein the liquid is a solution of a particleprecursor. The droplets are then heated to evaporate the liquid, reactthe precursors, and form solid particles.

The article entitled “Preparation of Fine Ni Particles by theSpray-Pyrolysis Technique and Their Film Forming Properties in the ThickFilm Method,” by Nagashima et al. (Journal of Materials Research, Vol.5, No. 12, December 1990) discloses the formation of nickel metalparticles by spray pyrolysis and the use of those particles for thickfilm pastes. Nickel particles are formed from nickel nitrate (Ni(NO₃)₂)and nickel chloride (NiCl₂) solutions. The solutions were atomized by anultrasonic atomizer and an N₂/H₂ carrier gas was utilized to carry thedroplets to a heated reaction zone. The reaction temperature was variedfrom 500° C. to 1600° C.

The article entitled “Preparation of Nickel Submicron Powder byUltrasonic Spray Pyrolysis” by Stopic et al. (The International Journalof Powder Metallurgy, Vol. 32, No. 1, 1996) discloses the formation ofnickel powder by spray pyrolysis. Powders were formed at temperatures of900° C. to 1000° C. under a reducing atmosphere. The authors state thatthe powders had a spherical morphology and were substantiallycrystalline.

There remains a need for nickel powders having a small particle size,narrow size distribution, high crystallinity (large crystals) andspherical morphology. It would be particularly advantageous if suchpowders could be produced in large quantities on a substantiallycontinuous basis.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a powder batchcomprising nickel particles is provided. The particles are substantiallyspherical, have a weight average particle size of not greater than about5 μm and a narrow particle size distribution and high crystallinity.

According to another embodiment of the present invention, a powder batchof metal alloy particles comprising nickel metal is provided wherein theparticles have a small particle size and a narrow particle sizedistribution. According to yet another embodiment of the presentinvention, a powder batch of coated nickel metal particles is provided.According to yet another embodiment of the present invention, a powderbatch of metal composite particles which include nickel metal and anon-metallic phase is provided.

The present invention also provides thick film paste compositionsincluding nickel particles, including coated nickel metal particles andcomposite nickel metal particles. The present invention also providesgreen bodies suitable for sintering to form multilayer ceramiccapacitors wherein the green bodies include a thick film pastecomposition comprising nickel metal particles.

The present invention is also directed to microelectronic devices,including multilayer ceramic capacitors, which incorporate the nickelmetal particles of the present invention.

The present invention is further directed to a method for the productionof nickel particles which generally includes generating an aerosol ofdroplets including a nickel metal precursor and moving the dropletsthrough a heating zone to form nickel particles. The method of thepresent invention is applicable to the formation of metal alloyparticles, composite particles and coated particles.

The present invention is also directed to devices fabricated from thenickel particles, including flat panel display devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process block diagram showing one embodiment of the processof the present invention.

FIG. 2 is a side view of a furnace and showing one embodiment of thepresent invention for sealing the end of a furnace tube.

FIG. 3 is a view of the side of an end cap that faces away from thefurnace shown in FIG. 2.

FIG. 4 is a view of the side of an end cap that faces toward the furnaceshown in FIG. 2.

FIG. 5 is a side view in cross section of one embodiment of aerosolgenerator of the present invention.

FIG. 6 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

FIG. 7 is a top view of a transducer mounting plate for a 400 transducerarray for use in an ultrasonic generator of the present invention.

FIG. 8 is a side view of the transducer mounting plate shown in FIG. 7.

FIG. 9 is a partial side view showing the profile of a single transducermounting receptacle of the transducer mounting plate shown in FIG. 7.

FIG. 10 is a partial side view in cross-section showing an alternativeembodiment for mounting an ultrasonic transducer.

FIG. 11 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

FIG. 12 is a top view of a liquid feed box having a bottom retainingplate to assist in retaining a separator for use in an aerosol generatorof the present invention.

FIG. 13 is a side view of the liquid feed box shown in FIG. 8.

FIG. 14 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

FIG. 15 shows a partial top view of gas tubes positioned in a liquidfeed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

FIG. 16 shows one embodiment for a gas distribution configuration forthe aerosol generator of the present invention.

FIG. 17 shows another embodiment for a gas distribution configurationfor the aerosol generator of the present invention.

FIG. 18 is a top view of one embodiment of a gas distribution plate/gastube assembly of the aerosol generator of the present invention.

FIG. 19 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 18.

FIG. 20 shows one embodiment for orienting a transducer in the aerosolgenerator of the present invention.

FIG. 21 is a top view of a gas manifold for distributing gas within anaerosol generator of the present invention.

FIG. 22 is a side view of the gas manifold shown in FIG. 21.

FIG. 23 is a top view of a generator lid of a hood design for use in anaerosol generator of the present invention.

FIG. 24 is a side view of the generator lid shown in FIG. 23.

FIG. 25 is a process block diagram of one embodiment in the presentinvention including an aerosol concentrator.

FIG. 26 is a top view in cross section of a virtual impactor that may beused for concentrating an aerosol according to the present invention.

FIG. 27 is a front view of an upstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 28 is a top view of the upstream plate assembly shown in FIG. 27.

FIG. 29 is a side view of the upstream plate assembly shown in FIG. 27.

FIG. 30 is a front view of a downstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 31 is a top view of the downstream plate assembly shown in FIG. 30.

FIG. 32 is a side view of the downstream plate assembly shown in FIG.30.

FIG. 33 is a process block diagram of one embodiment of the process ofthe present invention including a droplet classifier.

FIG. 34 is a top view in cross section of an impactor of the presentinvention for use in classifying an aerosol.

FIG. 35 is a front view of a flow control plate of the impactor shown inFIG. 34.

FIG. 36 is a front view of a mounting plate of the impactor shown inFIG. 34.

FIG. 37 is a front view of an impactor plate assembly of the impactorshown in FIG. 34.

FIG. 38 is a side view of the impactor plate assembly shown in FIG. 37.

FIG. 39 shows a side view in cross section of a virtual impactor incombination with an impactor of the present invention for concentratingand classifying droplets in an aerosol.

FIG. 40 is a process block diagram of one embodiment of the presentinvention including a particle cooler.

FIG. 41 is a top view of a gas quench cooler of the present invention.

FIG. 42 is an end view of the gas quench cooler shown in FIG. 41.

FIG. 43 is a side view of a perforated conduit of the quench coolershown in FIG. 41.

FIG. 44 is a side view showing one embodiment of a gas quench cooler ofthe present invention connected with a cyclone.

FIG. 45 is a process block diagram of one embodiment of the presentinvention including a particle coater.

FIG. 46 is a block diagram of one embodiment of the present inventionincluding a particle modifier.

FIG. 47 shows cross sections of various particle morphologies of somecomposite particles manufacturable according to the present invention.

FIG. 48 shows a side view of one embodiment of apparatus of the presentinvention including an aerosol generator, an aerosol concentrator, adroplet classifier, a furnace, a particle cooler, and a particlecollector.

FIG. 49 is a block diagram of one embodiment of the process of thepresent invention including the addition of a dry gas between theaerosol generator and the furnace.

FIG. 50 illustrates a schematic view of a microelectronic deviceaccording to an embodiment of the present invention.

FIG. 51 illustrates a top view of a microelectronic device according toan embodiment of the present invention.

FIG. 52 illustrates a schematic view of a multilayer ceramic capacitoraccording to an embodiment of the present invention.

FIG. 53 illustrates a schematic view of a plasma display panel accordingto an embodiment of the present invention.

FIG. 54 illustrates another view of a plasma display panel according toan embodiment of the present invention.

FIG. 55 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 56 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 57 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 58 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 59 illustrates an x-ray diffraction pattern of a nickel/palladiumalloy produced according to an embodiment of the present invention.

FIG. 60 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 61 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 62 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 63 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 64 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 65 illustrates a photomicrograph of a nickel metal powder accordingto an embodiment of the present invention.

FIG. 66 illustrates a photomicrograph of a nickel metal composite powderaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to nickel powders andmethods for producing the powders. The invention is also directed tonovel intermediate products and devices fabricated using the nickelpowders. As used herein, nickel powders or nickel particles are thosethat include nickel or a nickel-based compound such as pure nickelmetal, nickel metal alloys, intermetallic compounds and other nickelcompounds, composite particles, coated particles, and the like.

In one aspect, the present invention provides a method for preparing aparticulate product. A feed of liquid-containing, flowable medium,including at least one precursor for the desired particulate product, isconverted to aerosol form, with droplets of the medium being dispersedin and suspended by a carrier gas. Liquid from the droplets in theaerosol is then removed to permit formation in a dispersed state of thedesired particles. Typically, the feed precursor is pyrolyzed in afurnace to make the particles. In one embodiment, the particles aresubjected, while still in a dispersed state, to compositional orstructural modification, if desired. Compositional modification mayinclude, for example, coating the particles. Structural modification mayinclude, for example, crystallization, recrystallization ormorphological alteration of the particles. The term powder is often usedherein to refer to the particulate product of the present invention. Theuse of the term powder does not indicate, however, that the particulateproduct must be dry or in any particular environment. Although theparticulate product is typically manufactured in a dry state, theparticulate product may, after manufacture, be placed in a wetenvironment, such as in a paste or slurry.

The process of the present invention is particularly well suited for theproduction of particulate products of finely divided particles having asmall weight average size. In addition to making particles within adesired range of weight average particle size, with the presentinvention the particles may be produced with a desirably narrow sizedistribution, thereby providing size uniformity that is desired for manyapplications.

In addition to control over particle size and size distribution, themethod of the present invention provides significant flexibility forproducing particles of varying composition, crystallinity andmorphology. For example, the present invention may be used to producehomogeneous particles involving only a single phase or multi-phaseparticles including multiple phases. In the case of multi-phaseparticles, the phases may be present in a variety of morphologies. Forexample, one phase may be uniformly dispersed throughout a matrix ofanother phase. Alternatively, one phase may form an interior core whileanother phase forms a coating that surrounds the core. Othermorphologies are also possible, as discussed more fully below.

Referring now to FIG. 1, one embodiment of the process of the presentinvention is described. A liquid feed 102, including at least oneprecursor for the desired particles, and a carrier gas 104 are fed to anaerosol generator 106 where an aerosol 108 is produced. The aerosol 108is then fed to a furnace 110 where liquid in the aerosol 108 is removedto produce particles 112 that are dispersed in and suspended by gasexiting the furnace 110. The particles 112 are then collected in aparticle collector 114 to produce a particulate product 116.

As used herein, the liquid feed 102 is a feed that includes one or moreflowable liquids as the major constituent(s), such that the feed is aflowable medium. The liquid feed 102 need not comprise only liquidconstituents. The liquid feed 102 may comprise only constituents in oneor more liquid phase, or it may also include particulate materialsuspended in a liquid phase. The liquid feed 102 must, however, becapable of being atomized to form droplets of sufficiently small sizefor preparation of the aerosol 108. Therefore, if the liquid feed 102includes suspended particles, those particles should be relatively smallin relation to the size of droplets in the aerosol 108. Such suspendedparticles should typically be smaller than about 1 μm in size,preferably smaller than about 0.5 μm in size, and more preferablysmaller than about 0.3 μm in size and most preferably smaller than about0.1 μm in size. Most preferably, the suspended particles should be ableto form a colloid. The suspended particles could be finely dividedparticles, or could be agglomerate masses comprised of agglomeratedsmaller primary particles. For example, 0.5 μm particles could beagglomerates of nanometer-sized primary particles. When the liquid feed102 includes suspended particles, the particles typically comprise nogreater than about 25 to 50 weight percent of the liquid feed.

As noted, the liquid feed 102 includes at least one precursor forpreparation of the particles 112. The precursor may be a substance ineither a liquid or solid phase of the liquid feed 102. Frequently, theprecursor will be a material, such as a salt, dissolved in a liquidsolvent of the liquid feed 102. The precursor may undergo one or morechemical reactions in the furnace 110 to assist in production of theparticles 112. Alternatively, the precursor material may contribute toformation of the particles 112 without undergoing chemical reaction.This could be the case, for example, when the liquid feed 102 includes,as a precursor material, suspended particles that are not chemicallymodified in the furnace 110. In any event, the particles 112 comprise atleast one component originally contributed by the precursor.

The liquid feed 102 may include multiple precursor materials, which maybe present together in a single phase or separately in multiple phases.For example, the liquid feed 102 may include multiple precursors insolution in a single liquid vehicle. Alternatively, one precursormaterial could be in a solid particulate phase and a second precursormaterial could be in a liquid phase. Also, one precursor material couldbe in one liquid phase and a second precursor material could be in asecond liquid phase, such as could be the case when the liquid feed 102comprises an emulsion. Different components contributed by differentprecursors may be present in the particles together in a single materialphase, or the different components may be present in different materialphases when the particles 112 are composites of multiple phases.Specific examples of preferred precursor materials are discussed morefully below.

The carrier gas 104 may comprise any gaseous medium in which dropletsproduced from the liquid feed 102 may be dispersed in aerosol form.Also, the carrier gas 104 may be inert, in that the carrier gas 104 doesnot participate in formation of the particles 112. Alternatively, thecarrier gas may have one or more active component(s) that contribute toformation of the particles 112. In that regard, the carrier gas mayinclude one or more reactive components that react in the furnace 110 tocontribute to formation of the particles 112. Preferred carrier gascompositions are discussed more fully below.

The aerosol generator 106 atomizes the liquid feed 102 to form dropletsin a manner to permit the carrier gas 104 to sweep the droplets away toform the aerosol 108. The droplets comprise liquid from the liquid feed102. The droplets may, however, also include nonliquid material, such asone or more small particles held in the droplet by the liquid. Forexample, when the particles 112 are composite, or multi-phase,particles, one phase of the composite may be provided in the liquid feed102 in the form of suspended precursor particles and a second phase ofthe composite may be produced in the furnace 110 from one or moreprecursors in the liquid phase of the liquid feed 102. Furthermore theprecursor particles could be included in the liquid feed 102, andtherefore also in droplets of the aerosol 108, for the purpose only ofdispersing the particles for subsequent compositional or structuralmodification during or after processing in the furnace 110.

An important aspect of the present invention is generation of theaerosol 108 with droplets of a small average size, narrow sizedistribution. In this manner, the particles 112 may be produced at adesired small size with a narrow size distribution, which areadvantageous for many applications.

The aerosol generator 106 is capable of producing the aerosol 108 suchthat it includes droplets having a weight average size in a range havinga lower limit of about 1 μm and preferably about 2 μm; and an upperlimit of about 10 μm; preferably about 7 μm, more preferably about 5 μmand most preferably about 4 μm. A weight average droplet size in a rangeof from about 2 μm to about 4 μm is more preferred for mostapplications, with a weight average droplet size of about 3 μm beingparticularly preferred for some applications. The aerosol generator isalso capable of producing the aerosol 108 such that it includes dropletsin a narrow size distribution. Preferably, the droplets in the aerosolare such that at least about 70 percent (more preferably at least about80 weight percent and most preferably at least about 85 weight percent)of the droplets are smaller than about 10 μm and more preferably atleast about 70 weight percent (more preferably at least about 80 weightpercent and most preferably at least about 85 weight percent) aresmaller than about 5 μm. Furthermore, preferably no greater than about30 weight percent, more preferably no greater than about 25 weightpercent and most preferably no greater than about 20 weight percent, ofthe droplets in the aerosol 108 are larger than about twice the weightaverage droplet size.

Another important aspect of the present invention is that the aerosol108 may be generated without consuming excessive amounts of the carriergas 104. The aerosol generator 106 is capable of producing the aerosol108 such that it has a high loading, or high concentration, of theliquid feed 102 in droplet form. In that regard, the aerosol 108preferably includes greater than about 1×10⁶ droplets per cubiccentimeter of the aerosol 108, more preferably greater than about 5×10⁶droplets per cubic centimeter, still more preferably greater than about1×10⁷ droplets per cubic centimeter, and most preferably greater thanabout 5×10⁷ droplets per cubic centimeter. That the aerosol generator106 can produce such a heavily loaded aerosol 108 is particularlysurprising considering the high quality of the aerosol 108 with respectto small average droplet size and narrow droplet size distribution.Typically, droplet loading in the aerosol is such that the volumetricratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is largerthan about 0.04 milliliters of liquid feed 102 per liter of carrier gas104 in the aerosol 108, preferably larger than about 0.083 millilitersof liquid feed 102 per liter of carrier gas 104 in the aerosol 108, morepreferably larger than about 0.167 milliliters of liquid feed 102 perliter of carrier gas 104, still more preferably larger than about 0.25milliliters of liquid feed 102 per liter of carrier gas 104, and mostpreferably larger than about 0.333 milliliters of liquid feed 102 perliter of carrier gas 104.

This capability of the aerosol generator 106 to produce a heavily loadedaerosol 108 is even more surprising given the high droplet output rateof which the aerosol generator 106 is capable, as discussed more fullybelow. It will be appreciated that the concentration of liquid feed 102in the aerosol 108 will depend upon the specific components andattributes of the liquid feed 102 and, particularly, the size of thedroplets in the aerosol 108. For example, when the average droplet sizeis from about 2 μm to about 4 μm, the droplet loading is preferablylarger than about 0.15 milliliters of aerosol feed 102 per liter ofcarrier gas 104, more preferably larger than about 0.2 milliliters ofliquid feed 102 per liter of carrier gas 104, even more preferablylarger than about 0.2 milliliters of liquid feed 102 per liter ofcarrier gas 104, and most preferably larger than about 0.3 millilitersof liquid feed 102 per liter of carrier gas 104. When reference is madeherein to liters of carrier gas 104, it refers to the volume that thecarrier gas 104 would occupy under conditions of standard temperatureand pressure.

The furnace 110 may be any suitable device for heating the aerosol 108to evaporate liquid from the droplets of the aerosol 108 and therebypermit formation of the particles 112. The maximum average streamtemperature, or reaction temperature, refers to the maximum averagetemperature that an aerosol stream attains while flowing through thefurnace. This is typically determined by a temperature probe insertedinto the furnace. Preferred reaction temperatures according to thepresent invention are discussed more fully below.

Although longer residence times are possible, for many applications,residence time in the heating zone of the furnace 110 of shorter thanabout 4 seconds is typical, with shorter than about 2 seconds beingpreferred, shorter than about 1 second being more preferred, shorterthan about 0.5 second being even more preferred, and shorter than about0.2 second being most preferred. The residence time should be longenough, however, to assure that the particles 112 attain the desiredmaximum stream temperature for a given heat transfer rate. In thatregard, with extremely short residence times, higher furnacetemperatures could be used to increase the rate of heat transfer so longas the particles 112 attain a maximum temperature within the desiredstream temperature range. That mode of operation, however, is notpreferred. Also, it is preferred that, in most cases, the maximum streamtemperature not be attained in the furnace 110 until substantially atthe end of the heating zone in the furnace 110. For example, the heatingzone will often include a plurality of heating sections that are eachindependently controllable. The maximum stream temperature shouldtypically not be attained until the final heating section, and morepreferably until substantially at the end of the last heating section.This is important to reduce the potential for thermophoretic losses ofmaterial. Also, it is noted that as used herein, residence time refersto the actual time for a material to pass through the relevant processequipment. In the case of the furnace, this includes the effect ofincreasing velocity with gas expansion due to heating.

Typically, the furnace 110 will be a tube-shaped furnace, so that theaerosol 108 moving into and through the furnace does not encounter sharpedges on which droplets could collect. Loss of droplets to collection atsharp surfaces results in a lower yield of particles 112. Moreimportant, however, the accumulation of liquid at sharp edges can resultin re-release of undesirably large droplets back into the aerosol 108,which can cause contamination of the particulate product 116 withundesirably large particles. Also, over time, such liquid collection atsharp surfaces can cause fouling of process equipment, impairing processperformance.

The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, silica or alumina. Alternatively, the tube may be metallic.Advantages of using a metallic tube are low cost, ability to withstandsteep temperature gradients and large thermal shocks, machinability andweldability, and ease of providing a seal between the tube and otherprocess equipment. Disadvantages of using a metallic tube includelimited operating temperature and increased reactivity in some reactionsystems.

When a metallic tube is used in the furnace 110, it is preferably a highnickel content stainless steel alloy, such as a 330 stainless steel, ora nickel-based super alloy. As noted, one of the major advantages ofusing a metallic tube is that the tube is relatively easy to seal withother process equipment. In that regard, flange fittings may be weldeddirectly to the tube for connecting with other process equipment.Metallic tubes are generally preferred for making particles that do notrequire a maximum tube wall temperature of higher than about 1100° C.during particle manufacture.

When higher temperatures are required, ceramic tubes are typically used.One major problem with ceramic tubes, however, is that the tubes can bedifficult to seal with other process equipment, especially when the endsof the tubes are maintained at relatively high temperatures, as is oftenthe case with the present invention.

One configuration for sealing a ceramic tube is shown in FIGS. 2, 3 and4. The furnace 110 includes a ceramic tube 374 having an end cap 376fitted to each end of the tube 374, with a gasket 378 disposed betweencorresponding ends of the ceramic tube 374 and the end caps 376. Thegasket may be of any suitable material for sealing at the temperatureencountered at the ends of the tubes 374. Examples of gasket materialsfor sealing at temperatures below about 250° C. include silicone,TEFLON™ and VITON™. Examples of gasket materials for higher temperaturesinclude graphite, ceramic paper, thin sheet metal, and combinationsthereof.

Tension rods 380 extend over the length of the furnace 110 and throughrod holes 382 through the end caps 376. The tension rods 380 are held intension by the force of springs 384 bearing against bearing plates 386and the end caps 376. The tube 374 is, therefore, in compression due tothe force of the springs 384. The springs 384 may be compressed anydesired amount to form a seal between the end caps 376 and the ceramictube 374 through the gasket 378. As will be appreciated, by using thesprings 384, the tube 374 is free to move to some degree as it expandsupon heating and contracts upon cooling. To form the seal between theend caps 376 and the ceramic tube 374, one of the gaskets 378 is seatedin a gasket seat 388 on the side of each end cap 376 facing the tube374. A mating face 390 on the side of each of the end caps 376 facesaway from the tube 374, for mating with a flange surface for connectionwith an adjacent piece of equipment.

Also, although the present invention is described with primary referenceto a furnace reactor, which is preferred, it should be recognized that,except as noted, any other thermal reactor, including a flame reactor ora plasma reactor, could be used instead. A furnace reactor is, however,preferred, because of the generally even heating characteristic of afurnace for attaining a uniform stream temperature.

The particle collector 114, may be any suitable apparatus for collectingparticles 112 to produce the particulate product 116. One preferredembodiment of the particle collector 114 uses one or more filter toseparate the particles 112 from gas. Such a filter may be of any type,including a bag filter. Another preferred embodiment of the particlecollector uses one or more cyclone to separate the particles 112. Otherapparatus that may be used in the particle collector 114 includes anelectrostatic precipitator. Also, collection should normally occur at atemperature above the condensation temperature of the gas stream inwhich the particles 112 are suspended. Also, collection should normallybe at a temperature that is low enough to prevent significantagglomeration of the particles 112.

Of significant importance to the operation of the process of the presentinvention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 5, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 5, includesforty-nine transducers in a 7×7 array. The array configuration is asshown in FIG. 6, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

With continued reference to FIG. 5, a separator 126, in spaced relationto the transducer discs 120, is retained between a bottom retainingplate 128 and a top retaining plate 130. Gas delivery tubes 132 areconnected to gas distribution manifolds 134, which have gas deliveryports 136. The gas distribution manifolds 134 are housed within agenerator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

During operation of the aerosol generator 106, as shown in FIG. 5, thetransducer discs 120 are activated by the transducer driver 144 via theelectrical cables 146. The transducers preferably vibrate at a frequencyof from about 1 MHz to about 5 MHz, more preferably from about 1.5 MHzto about 3 MHz. Frequently used frequencies are at about 1.6 MHz andabout 2.4 MHz. Furthermore, all of the transducer discs 110 should beoperating at substantially the same frequency when an aerosol with anarrow droplet size distribution is desired. This is important becausecommercially available transducers can vary significantly in thickness,sometimes by as much as 10%. It is preferred, however, that thetransducer discs 120 operate at frequencies within a range of 5% aboveand below the median transducer frequency, more preferably within arange of 2.5%, and most preferably within a range of 1%. This can beaccomplished by careful selection of the transducer discs 120 so thatthey all preferably have thicknesses within 5% of the median transducerthickness, more preferably within 2.5%, and most preferably within 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flowchannels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water, enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 causeatomization cones 162 to develop in the liquid feed 102 at locationscorresponding with the transducer discs 120. Carrier gas 104 isintroduced into the gas delivery tubes 132 and delivered to the vicinityof the atomization cones 162 via gas delivery ports 136. Jets of carriergas exit the gas delivery ports 136 in a direction so as to impinge onthe atomization cones 162, thereby sweeping away atomized droplets ofthe liquid feed 102 that are being generated from the atomization cones162 and creating the aerosol 108, which exits the aerosol generator 106through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of theaerosol generator 106. The embodiment of the aerosol generator 106 shownin FIG. 5 includes two gas exit ports per atomization cone 162, with thegas ports being positioned above the liquid medium 102 over troughs thatdevelop between the atomization cones 162, such that the exiting carriergas 104 is horizontally directed at the surface of the atomization cones162, thereby efficiently distributing the carrier gas 104 to criticalportions of the liquid feed 102 for effective and efficient sweepingaway of droplets as they form about the ultrasonically energizedatomization cones 162. Furthermore, it is preferred that at least aportion of the opening of each of the gas delivery ports 136, throughwhich the carrier gas exits the gas delivery tubes, should be locatedbelow the top of the atomization cones 162 at which the carrier gas 104is directed. This relative placement of the gas delivery ports 136 isvery important to efficient use of carrier gas 104. Orientation of thegas delivery ports 136 is also important. Preferably, the gas deliveryports 136 are positioned to horizontally direct jets of the carrier gas104 at the atomization cones 162. The aerosol generator 106 permitsgeneration of the aerosol 108 with heavy loading with droplets of thecarrier liquid 102, unlike aerosol generator designs that do notefficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in FIG.5, is the use of the separator 126, which protects the transducer discs120 from direct contact with the liquid feed 102, which is often highlycorrosive. The height of the separator 126 above the top of thetransducer discs 120 should normally be kept as small as possible, andis often in the range of from about 1 centimeter to about 2 centimeters.The top of the liquid feed 102 in the flow channels above the tops ofthe ultrasonic transducer discs 120 is typically in a range of fromabout 2 centimeters to about 5 centimeters, whether or not the aerosolgenerator includes the separator 126, with a distance of about 3 to 4centimeters being preferred. Although the aerosol generator 106 could bemade without the separator 126, in which case the liquid feed 102 wouldbe in direct contact with the transducer discs 120, the highly corrosivenature of the liquid feed 102 can often cause premature failure of thetransducer discs 120. The use of the separator 126, in combination withuse of the ultrasonically transmissive fluid in the water bath volume156 to provide ultrasonic coupling, significantly extending the life ofthe ultrasonic transducers 120. One disadvantage of using the separator126, however, is that the rate of droplet production from theatomization cones 162 is reduced, often by a factor of two or more,relative to designs in which the liquid feed 102 is in direct contactwith the ultrasonic transducer discs 102. Even with the separator 126,however, the aerosol generator 106 used with the present invention iscapable of producing a high quality aerosol with heavy droplet loading,as previously discussed. Suitable materials for the separator 126include, for example, polyamides (such as Kapton™ membranes from DuPont)and other polymer materials, glass, and plexiglass. The mainrequirements for the separator 126 are that it be ultrasonicallytransmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such coating typically significantly extends transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

One surprising finding with operation of the aerosol generator 106 ofthe present invention is that the droplet loading in the aerosol may beaffected by the temperature of the liquid feed 102. It has been foundthat when the liquid feed 102 includes an aqueous liquid at an elevatedtemperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer disks 120 are preferably also at an elevatedtemperature in the ranges just discussed for the liquid feed 102.

The design for the aerosol generator 106 based on an array of ultrasonictransducers is versatile and is easily modified to accommodate differentgenerator sizes for different specialty applications. The aerosolgenerator 106 may be designed to include a plurality of ultrasonictransducers in any convenient number. Even for smaller scale production,however, the aerosol generator 106 preferably has at least nineultrasonic transducers, more preferably at least 16 ultrasonictransducers, and even more preferably at least 25 ultrasonictransducers. For larger scale production, however, the aerosol generator106 includes at least 40 ultrasonic transducers, more preferably atleast 100 ultrasonic transducers, and even more preferably at least 400ultrasonic transducers. In some large volume applications, the aerosolgenerator may have at least 1000 ultrasonic transducers.

FIGS. 7–24 show component designs for an aerosol generator 106 includingan array of 400 ultrasonic transducers. Referring first to FIGS. 7 and8, the transducer mounting plate 124 is shown with a design toaccommodate an array of 400 ultrasonic transducers, arranged in foursubarrays of 100 ultrasonic transducers each. The transducer mountingplate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 5.

As shown in FIGS. 7 and 8, four hundred transducer mounting receptacles174 are provided in the transducer mounting plate 124 for mountingultrasonic transducers for the desired array. With reference to FIG. 9,the profile of an individual transducer mounting receptacle 174 isshown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 5.

A preferred transducer mounting configuration, however, is shown in FIG.10 for another configuration for the transducer mounting plate 124. Asseen in FIG. 10, an ultrasonic transducer disc 120 is mounted to thetransducer mounting plate 124 by use of a compression screw 177 threadedinto a threaded receptacle 179. The compression screw 177 bears againstthe ultrasonic transducer disc 120, causing an o-ring 181, situated inan o-ring seat 182 on the transducer mounting plate, to be compressed toform a seal between the transducer mounting plate 124 and the ultrasonictransducer disc 120. This type of transducer mounting is particularlypreferred when the ultrasonic transducer disc 120 includes a protectivesurface coating, as discussed previously, because the seal of the o-ringto the ultrasonic transducer disc 120 will be inside of the outer edgeof the protective seal, thereby preventing liquid from penetrating underthe protective surface coating from the edges of the ultrasonictransducer disc 120.

Referring now to FIG. 11, the bottom retaining plate 128 for a 400transducer array is shown having a design for mating with the transducermounting plate 124 (shown in FIGS. 7–8). The bottom retaining plate 128has eighty openings 184, arranged in four subgroups 186 of twentyopenings 184 each. Each of the openings 184 corresponds with five of thetransducer mounting receptacles 174 (shown in FIGS. 7 and 8) when thebottom retaining plate 128 is mated with the transducer mounting plate124 to create a volume for a water bath between the transducer mountingplate 124 and the bottom retaining plate 128. The openings 184,therefore, provide a pathway for ultrasonic signals generated byultrasonic transducers to be transmitted through the bottom retainingplate.

Referring now to FIGS. 12 and 13, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 11), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 12 and 13 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 11). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

Referring now to FIGS. 12–14, a plurality of gas tube feed-through holes202 extend through the vertically extending walls 192 to either side ofthe assembly including the feed inlet 148 and feed outlet 152 of theliquid feed box 190. The gas tube feed-through holes 202 are designed topermit insertion therethrough of gas tubes 208 of a design as shown inFIG. 14. When the aerosol generator 106 is assembled, a gas tube 208 isinserted through each of the gas tube feed-through holes 202 so that gasdelivery ports 136 in the gas tube 208 will be properly positioned andaligned adjacent the openings 194 in the top retaining plate 130 fordelivery of gas to atomization cones that develop in the liquid feed box190 during operation of the aerosol generator 106. The gas deliveryports 136 are typically holes having a diameter of from about 1.5millimeters to about 3.5 millimeters.

Referring now to FIG. 15, a partial view of the liquid feed box 190 isshown with gas tubes 208A, 208B and 208C positioned adjacent to theopenings 194 through the top retaining plate 130. Also shown in FIG. 15are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator 106 is assembled. As seen in FIG. 15,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 15, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 5, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 15 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

Referring now to FIG. 16, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 14. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

An alternative, and preferred, flow for carrier gas 104 is shown in FIG.17. As shown in FIG. 17, carrier gas 104 is delivered from only one sideof each of the gas tubes 208. This results in a sweep of carrier gasfrom all of the gas tubes 208 toward a central area 212. This results ina more uniform flow pattern for aerosol generation that maysignificantly enhance the efficiency with which the carrier gas 104 isused to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosolgenerator 106 is shown in FIGS. 18 and 19. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would be mounted above theliquid feed, with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 19,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 18 and 19 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonictransducers at a slight angle and directing the carrier gas at resultingatomization cones such that the atomization cones are tilting in thesame direction as the direction of flow of carrier gas. Referring toFIG. 20, an ultrasonic transducer disc 120 is shown. The ultrasonictransducer disc 120 is tilted at a tilt angle 114 (typically less than10 degrees), so that the atomization cone 162 will also have a tilt. Itis preferred that the direction of flow of the carrier gas 104 directedat the atomization cone 162 is in the same direction as the tilt of theatomization cone 162.

Referring now to FIGS. 21 and 22, a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 14). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 21 and 22, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

Referring now to FIGS. 23 and 24, the generator lid 140 is shown for a400 transducer array design. The generator lid 140 mates with and coversthe liquid feed box 190 (shown in FIGS. 12 and 13). The generator lid140, as shown in FIGS. 23 and 24, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

Although the aerosol generator 106 produces a high quality aerosol 108having a high droplet loading, it is often desirable to furtherconcentrate the aerosol 108 prior to introduction into the furnace 110.Referring now to FIG. 25, a process flow diagram is shown for oneembodiment of the present invention involving such concentration of theaerosol 108. As shown in FIG. 25, the aerosol 108 from the aerosolgenerator 106 is sent to an aerosol concentrator 236 where excesscarrier gas 238 is withdrawn from the aerosol 108 to produce aconcentrated aerosol 240, which is then fed to the furnace 110.

The aerosol concentrator 236 typically includes one or more virtualimpactors capable of concentrating droplets in the aerosol 108 by afactor of greater than about 2, preferably by a factor of greater thanabout 5, and more preferably by a factor of greater than about 10, toproduce the concentrated aerosol 240. According to the presentinvention, the concentrated aerosol 240 should typically contain greaterthan about 1×10⁷ droplets per cubic centimeter, and more preferably fromabout 5×10⁷ to about 5×10⁸ droplets per cubic centimeter. Aconcentration of about 1×10⁸ droplets per cubic centimeter of theconcentrated aerosol is particularly preferred, because when theconcentrated aerosol 240 is loaded more heavily than that, then thefrequency of collisions between droplets becomes large enough to impairthe properties of the concentrated aerosol 240, resulting in potentialcontamination of the particulate product 116 with an undesirably largequantity of over-sized particles. For example, if the aerosol 108 has aconcentration of about 1×10⁷ droplets per cubic centimeter, and theaerosol concentrator 236 concentrates droplets by a factor of 10, thenthe concentrated aerosol 240 will have a concentration of about 1×10⁸droplets per cubic centimeter. Stated another way, for example, when theaerosol generator generates the aerosol 108 with a droplet loading ofabout 0.167 milliliters liquid feed 102 per liter of carrier gas 104,the concentrated aerosol 240 would be loaded with about 1.67 millilitersof liquid feed 102 per liter of carrier gas 104, assuming the aerosol108 is concentrated by a factor of 10.

Having a high droplet loading in aerosol feed to the furnace providesthe important advantage of reducing the heating demand on the furnace110 and the size of flow conduits required through the furnace. Also,other advantages of having a dense aerosol include a reduction in thedemands on cooling and particle collection components, permittingsignificant equipment and operational savings. Furthermore, as systemcomponents are reduced in size, powder holdup within the system isreduced, which is also desirable. Concentration of the aerosol streamprior to entry into the furnace 110, therefore, provides a substantialadvantage relative to processes that utilize less concentrated aerosolstreams.

The excess carrier gas 238 that is removed in the aerosol concentrator236 typically includes extremely small droplets that are also removedfrom the aerosol 108. Preferably, the droplets removed with the excesscarrier gas 238 have a weight average size of smaller than about 1.5 μm,and more preferably smaller than about 1 μm and the droplets retained inthe concentrated aerosol 240 have an average droplet size of larger thanabout 2 μm. For example, a virtual impactor sized to treat an aerosolstream having a weight average droplet size of about three μm might bedesigned to remove with the excess carrier gas 238 most droplets smallerthan about 1.5 μm in size. Other designs are also possible. When usingthe aerosol generator 106 with the present invention, however, the lossof these very small droplets in the aerosol concentrator 236 willtypically constitute no more than about 10 percent by weight, and morepreferably no more than about 5 percent by weight, of the dropletsoriginally in the aerosol stream that is fed to the concentrator 236.Although the aerosol concentrator 236 is useful in some situations, itis normally not required with the process of the present invention,because the aerosol generator 106 is capable, in most circumstances, ofgenerating an aerosol stream that is sufficiently dense. So long as theaerosol stream coming out of the aerosol generator 102 is sufficientlydense, it is preferred that the aerosol concentrator not be used. It isa significant advantage of the present invention that the aerosolgenerator 106 normally generates such a dense aerosol stream that theaerosol concentrator 236 is not needed. Therefore, the complexity ofoperation of the aerosol concentrator 236 and accompanying liquid lossesmay typically be avoided.

It is important that the aerosol stream (whether it has beenconcentrated or not) that is fed to the furnace 110 have a high dropletflow rate and high droplet loading as would be required for mostindustrial applications. With the present invention, the aerosol streamfed to the furnace preferably includes a droplet flow of greater thanabout 0.5 liters per hour, more preferably greater than about 2 litersper hour, still more preferably greater than about 5 liters per hour,even more preferably greater than about 10 liters per hour, particularlygreater than about 50 liters per hour and most preferably greater thanabout 100 liters per hour; and with the droplet loading being typicallygreater than about 0.04 milliliters of droplets per liter of carriergas, preferably greater than about 0.083 milliliters of droplets perliter of carrier gas 104, more preferably greater than about 0.167milliliters of droplets per liter of carrier gas 104, still morepreferably greater than about 0.25 milliliters of droplets per liter ofcarrier gas 104, particularly greater than about 0.33 milliliters ofdroplets per liter of carrier gas 104 and most preferably greater thanabout 0.83 milliliters of droplets per liter of carrier gas 104.

One embodiment of a virtual impactor that could be used as the aerosolconcentrator 236 will now be described with reference to FIGS. 26–32. Avirtual impactor 246 includes an upstream plate assembly 248 (detailsshown in FIGS. 27–29) and a downstream plate assembly 250 (details shownin FIGS. 25–32), with a concentrating chamber 262 located between theupstream plate assembly 248 and the downstream plate assembly 250.

Through the upstream plate assembly 248 are a plurality of verticallyextending inlet slits 254. The downstream plate assembly 250 includes aplurality of vertically extending exit slits 256 that are in alignmentwith the inlet slits 254. The exit slits 256 are, however, slightlywider than the inlet slits 254. The downstream plate assembly 250 alsoincludes flow channels 258 that extend substantially across the width ofthe entire downstream plate assembly 250, with each flow channel 258being adjacent to an excess gas withdrawal port 260.

During operation, the aerosol 108 passes through the inlet slits 254 andinto the concentrating chamber 262. Excess carrier gas 238 is withdrawnfrom the concentrating chamber 262 via the excess gas withdrawal ports260. The withdrawn excess carrier gas 238 then exits via a gas duct port264. That portion of the aerosol 108 that is not withdrawn through theexcess gas withdrawal ports 260 passes through the exit slits 256 andthe flow channels 258 to form the concentrated aerosol 240. Thosedroplets passing across the concentrating chamber 262 and through theexit slits 256 are those droplets of a large enough 110 size to havesufficient momentum to resist being withdrawn with the excess carriergas 238.

As seen best in FIGS. 27–32, the inlet slits 254 of the upstream plateassembly 248 include inlet nozzle extension portions 266 that extendoutward from the plate surface 268 of the upstream plate assembly 248.The exit slits 256 of the downstream plate assembly 250 include exitnozzle extension portions 270 extending outward from a plate surface 272of the downstream plate assembly 250. These nozzle extension portions266 and 270 are important for operation of the virtual impactor 246,because having these nozzle extension portions 266 and 270 permits avery close spacing to be attained between the inlet slits 254 and theexit slits 256 across the concentrating chamber 262, while alsoproviding a relatively large space in the concentrating chamber 262 tofacilitate efficient removal of the excess carrier gas 238.

Also as best seen in FIGS. 27–32, the inlet slits 254 have widths thatflare outward toward the side of the upstream plate assembly 248 that isfirst encountered by the aerosol 108 during operation. This flaredconfiguration reduces the sharpness of surfaces encountered by theaerosol 108, reducing the loss of aerosol droplets and potentialinterference from liquid buildup that could occur if sharp surfaces werepresent. Likewise, the exit slits 256 have a width that flares outwardtowards the flow channels 258, thereby allowing the concentrated aerosol240 to expand into the flow channels 258 without encountering sharpedges that could cause problems.

As noted previously, both the inlet slits 254 of the upstream plateassembly 248 and the exit slits 256 of the downstream plate assembly 250are vertically extending. This configuration is advantageous forpermitting liquid that may collect around the inlet slits 254 and theexit slits 256 to drain away. The inlet slits 254 and the exit slits 256need not, however, have a perfectly vertical orientation. Rather, it isoften desirable to slant the slits backward (sloping upward and away inthe direction of flow) by about five to ten degrees relative tovertical, to enhance draining of liquid off of the upstream plateassembly 248 and the downstream plate assembly 250. This drainagefunction of the vertically extending configuration of the inlet slits254 and the outlet slits 256 also inhibits liquid build-up in thevicinity of the inlet slits 248 and the exit slits 250, which liquidbuild-up could result in the release of undesirably large droplets intothe concentrated aerosol 240.

As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. It has beenfound, however, that for many applications the process of the presentinvention is significantly enhanced by further classifying by size thedroplets in the aerosol 108 prior to introduction of the droplets intothe furnace 110. In this manner, the size and size distribution ofparticles in the particulate product 116 are further controlled.

Referring now to FIG. 33, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 33, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

Any suitable droplet classifier may be used for removing droplets abovea predetermined size. For example, a cyclone could be used to removeover-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 34–38.

As seen in FIG. 34, an impactor 288 has disposed in a flow conduit 286 aflow control plate 290 and an impactor plate assembly 292. The flowcontrol plate 290 is conveniently mounted on a mounting plate 294.

The flow control plate 290 is used to channel the flow of the aerosolstream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 is shown in FIG. 35. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

Details of the mounting plate 294 are shown in FIG. 36. The mountingplate 294 has a mounting flange 298 with a large diameter flow opening300 passing therethrough to permit access of the aerosol 108 to the flowports 296 of the flow control plate 290 (shown in FIG. 35).

Referring now to FIGS. 37 and 38, one embodiment of an impactor plateassembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

During operation of the impactor 288, the aerosol 108 from the aerosolgenerator 106 passes through the upstream flow control plate 290. Mostof the droplets in the aerosol navigate around the impactor plate 302and exit the impactor 288 through the downstream flow control plate 290in the classified aerosol 282. Droplets in the aerosol 108 that are toolarge to navigate around the impactor plate 302 will impact on theimpactor plate 302 and drain through the drain 296 to be collected withthe drained liquid 284 (as shown in FIG. 34).

The configuration of the impactor plate 302 shown in FIG. 33 representsonly one of many possible configurations for the impactor plate 302. Forexample, the impactor 288 could include an upstream flow control plate290 having vertically extending flow slits therethrough that are offsetfrom vertically extending flow slits through the impactor plate 302,such that droplets too large to navigate the change in flow due to theoffset of the flow slits between the flow control plate 290 and theimpactor plate 302 would impact on the impactor plate 302 to be drainedaway. Other designs are also possible.

In a preferred embodiment of the present invention, the dropletclassifier 280 is typically designed to remove droplets from the aerosol108 that are larger than about 15 μm in size, more preferably to removedroplets larger than about 10 μm in size, even more preferably to removedroplets of a size larger than about 8 μm in size and most preferably toremove droplets larger than about 5 μm in size. The dropletclassification size in the droplet classifier is preferably smaller thanabout 15 μm, more preferably smaller than about 10 μm, even morepreferably smaller than about 8 μm and most preferably smaller thanabout 5 μm. The classification size, also called the classification cutpoint, is that size at which half of the droplets of that size areremoved and half of the droplets of that size are retained. Dependingupon the specific application, however, the droplet classification sizemay be varied, such as by changing the spacing between the impactorplate 302 and the flow control plate 290 or increasing or decreasingaerosol velocity through the jets in the flow control plate 290. Becausethe aerosol generator 106 of the present invention initially produces ahigh quality aerosol 108, having a relatively narrow size distributionof droplets, typically less than about 30 weight percent of liquid feed102 in the aerosol 108 is removed as the drain liquid 284 in the dropletclassifier 288, with preferably less than about 25 weight percent beingremoved, even more preferably less than about 20 weight percent beingremoved and most preferably less than about 15 weight percent beingremoved. Minimizing the removal of liquid feed 102 from the aerosol 108is particularly important for commercial applications to increase theyield of high quality particulate product 116. It should be noted,however, that because of the superior performance of the aerosolgenerator 106, it is frequently not required to use an impactor or otherdroplet classifier to obtain a desired absence of oversize droplets tothe furnace. This is a major advantage, because the added complexity andliquid losses accompanying use of an impactor may often be avoided withthe process of the present invention.

Sometimes it is desirable to use both the aerosol concentrator 236 andthe droplet classifier 280 to produce an extremely high quality aerosolstream for introduction into the furnace for the production of particlesof highly controlled size and size distribution. Referring now to FIG.39, one embodiment of the present invention is shown incorporating boththe virtual impactor 246 and the impactor 288. Basic components of thevirtual impactor 246 and the impactor 288, as shown in FIG. 39, aresubstantially as previously described with reference to FIGS. 26–38. Asseen in FIG. 39, the aerosol 108 from the aerosol generator 106 is fedto the virtual impactor 246 where the aerosol stream is concentrated toproduce the concentrated aerosol 240. The concentrated aerosol 240 isthen fed to the impactor 288 to remove large droplets therefrom andproduce the classified aerosol 282, which may then be fed to the furnace110. Also, it should be noted that by using both a virtual impactor andan impactor, both undesirably large and undesirably small droplets areremoved, thereby producing a classified aerosol with a very narrowdroplet size distribution. Also, the order of the aerosol concentratorand the aerosol classifier could be reversed, so that the aerosolconcentrator 236 follows the aerosol classifier 280.

One important feature of the design shown in FIG. 39 is theincorporation of drains 310, 312, 314, 316 and 296 at strategiclocations. These drains are extremely important for industrial-scaleparticle production because buildup of liquid in the process equipmentcan significantly impair the quality of the particulate product 116 thatis produced. In that regard, drain 310 drains liquid away from the inletside of the first plate assembly 248 of the virtual impactor 246. Drain312 drains liquid away from the inside of the concentrating chamber 262in the virtual impactor 246 and drain 314 removes liquid that depositsout of the excess carrier gas 238. Drain 316 removes liquid from thevicinity of the inlet side of the flow control plate 290 of theimpactor, while the drain 296 removes liquid from the vicinity of theimpactor plate 302. Without these drains 310, 312, 314, 316 and 296, theperformance of the apparatus shown in FIG. 39 would be significantlyimpaired. All liquids drained in the drains 310, 312, 314, 316 and 296may advantageously be recycled for use to prepare the liquid feed 102.

With some applications of the process of the present invention, it maybe possible to collect the particles 112 directly from the output of thefurnace 110. More often, however, it will be desirable to cool theparticles 112 exiting the furnace 110 prior to collection of theparticles 112 in the particle collector 114. Referring now to FIG. 40,one embodiment of the process of the present invention is shown in whichthe particles 112 exiting the furnace 110 are sent to a particle cooler320 to produce a cooled particle stream 322, which is then feed to theparticle collector 114. Although the particle cooler 320 may be anycooling apparatus capable of cooling the particles 112 to the desiredtemperature for introduction into the particle collector 114,traditional heat exchanger designs are not preferred. This is because atraditional heat exchanger design ordinarily directly subjects theaerosol stream, in which the hot particles 112 are suspended, to coolsurfaces. In that situation, significant losses of the particles 112occur due to thermophoretic deposition of the hot particles 112 on thecool surfaces of the heat exchanger. According to the present invention,a gas quench apparatus is provided for use as the particle cooler 320that significantly reduces thermophoretic losses compared to atraditional heat exchanger.

Referring now to FIGS. 41–43, one embodiment of a gas quench cooler 330is shown. The gas quench cooler includes a perforated conduit 332 housedinside of a cooler housing 334 with an annular space 336 located betweenthe cooler housing 334 and the perforated conduit 332. In fluidcommunication with the annular space 336 is a quench gas inlet box 338,inside of which is disposed a portion of an aerosol outlet conduit 340.The perforated conduit 332 extends between the aerosol outlet conduit340 and an aerosol inlet conduit 342. Attached to an opening into thequench gas inlet box 338 are two quench gas feed tubes 344. Referringspecifically to FIG. 43, the perforated tube 332 is shown. Theperforated tube 332 has a plurality of openings 345. The openings 345,when the perforated conduit 332 is assembled into the gas quench cooler330, permit the flow of quench gas 346 from the annular space 336 intothe interior space 348 of the perforated conduit 332. Although theopenings 345 are shown as being round holes, any shape of opening couldbe used, such as slits. Also, the perforated conduit 332 could be aporous screen. Two heat radiation shields 347 prevent downstream radiantheating from the furnace. In most instances, however, it will not benecessary to include the heat radiation shields 347, because downstreamradiant heating from the furnace is normally not a significant problem.Use of the heat radiation shields 347 is not preferred due toparticulate losses that accompany their use.

With continued reference to FIGS. 41–43, operation of the gas quenchcooler 330 will now be described. During operation, the particles 112,carried by and dispersed in a gas stream, enter the gas quench cooler330 through the aerosol inlet conduit 342 and flow into the interiorspace 348 of perforated conduit 332. Quench gas 346 is introducedthrough the quench gas feed tubes 344 into the quench gas inlet box 338.Quench gas 346 entering the quench gas inlet box 338 encounters theouter surface of the aerosol outlet conduit 340, forcing the quench gas346 to flow, in a spiraling, swirling manner, into the annular space336, where the quench gas 346 flows through the openings 345 through thewalls of the perforated conduit 332. Preferably, the gas 346 retainssome swirling motion even after passing into the interior space 348. Inthis way, the particles 112 are quickly cooled with low losses ofparticles to the walls of the gas quench cooler 330. In this manner, thequench gas 346 enters in a radial direction into the interior space 348of the perforated conduit 332 around the entire periphery, orcircumference, of the perforated conduit 332 and over the entire lengthof the perforated conduit 332. The cool quench gas 346 mixes with andcools the hot particles 112, which then exit through the aerosol outletconduit 340 as the cooled particle stream 322. The cooled particlestream 322 can then be sent to the particle collector 114 for particlecollection. The temperature of the cooled particle stream 322 iscontrolled by introducing more or less quench gas. Also, as shown inFIG. 41, the quench gas 346 is fed into the quench cooler 330 in counterflow to flow of the particles. Alternatively, the quench cooler could bedesigned so that the quench gas 346 is fed into the quench cooler inconcurrent flow with the flow of the particles 112. The amount of quenchgas 346 fed to the gas quench cooler 330 will depend upon the specificmaterial being made and the specific operating conditions. The quantityof quench gas 346 used, however, must be sufficient to reduce thetemperature of the aerosol steam including the particles 112 to thedesired temperature. Typically, the particles 112 are cooled to atemperature at least below about 200° C., and often lower. The onlylimitation on how much the particles 112 are cooled is that the cooledparticle stream 322 must be at a temperature that is above thecondensation temperature for water as another condensible vapor in thestream. The temperature of the cooled particle stream 322 is often at atemperature of from about 50° C. to about 120° C.

Because of the entry of quench gas 346 into the interior space 348 ofthe perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

As seen in FIGS. 41–43, the gas quench cooler 330 includes a flow pathfor the particles 112 through the gas quench cooler of a substantiallyconstant cross-sectional shape and area. Preferably, the flow paththrough the gas quench cooler 330 will have the same cross-sectionalshape and area as the flow path through the furnace 110 and through theconduit delivering the aerosol 108 from the aerosol generator 106 to thefurnace 110. In one embodiment, however, it may be necessary to reducethe cross-sectional area available for flow prior to the particlecollector 114. This is the case, for example, when the particlecollector includes a cyclone for separating particles in the cooledparticle stream 322 from gas in the cooled particle stream 322. This isbecause of the high inlet velocity requirements into cyclone separators.

Referring now to FIG. 44, one embodiment of the gas quench cooler 330 isshown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for powder having a weight averagesize of larger than about 1 μm, although a series of cyclones maysometimes be needed to get the desired degree of separation. Cycloneseparation is particularly preferred for powders having a weight averagesize of larger than about 1.5 μm. Also, cyclone separation is bestsuited for high density materials. Preferably, when particles areseparated using a cyclone, the particles are of a composition withspecific gravity of greater than about 5.

In an additional embodiment, the process of the present invention canalso incorporate compositional modification of the particles 112 exitingthe furnace. Most commonly, the compositional modification will involveforming on the particles 112 a material phase that is different thanthat of the particles 112, such as by coating the particles 112 with acoating material. One embodiment of the process of the present inventionincorporating particle coating is shown in FIG. 45. As shown in FIG. 45,the particles 112 exiting from the furnace 110 go to a particle coater350 where a coating is placed over the outer surface of the particles112 to form coated particles 352, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Coatingmethodologies employed in the particle coater 350 are discussed in moredetail below.

With continued reference primarily to FIG. 45, in a preferredembodiment, when the particles 112 are coated according to the processof the present invention, the particles 112 are also manufactured viathe aerosol process of the present invention, as previously described.The process of the present invention can, however, be used to coatparticles that have been premanufactured by a different process, such asby a liquid precipitation route When coating particles that have beenpremanufactured by a different route, such as by liquid precipitation,it is preferred that the particles remain in a dispersed state from thetime of manufacture to the time that the particles are introduced inslurry form into the aerosol generator 106 for preparation of theaerosol 108 to form the dry particles 112 in the furnace 110, whichparticles 112 can then be coated in the particle coater 350. Maintainingparticles in a dispersed state from manufacture through coating avoidsproblems associated with agglomeration and redispersion of particles ifparticles must be redispersed in the liquid feed 102 for feed to theaerosol generator 106. For example, for particles originallyprecipitated from a liquid medium, the liquid medium containing thesuspended precipitated particles could be used to form the liquid feed102 to the aerosol generator 106. It should be noted that the particlecoater 350 could be an integral extension of the furnace 110 or could bea separate piece of equipment.

In a further embodiment of the present invention, following preparationof the particles 112 in the furnace 110, the particles 112 may then bestructurally modified to impart desired physical properties prior toparticle collection. Referring now to FIG. 46, one embodiment of theprocess of the present invention is shown including such structuralparticle modification. The particles 112 exiting the furnace 110 go to aparticle modifier 360 where the particles are structurally modified toform modified particles 362, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Theparticle modifier 360 is typically a furnace, such as an annealingfurnace, which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theparticles 112. The particle modifier 360, therefore, typically providesa temperature controlled environment and necessary residence time toeffect the desired structural modification of the particles 112.

The structural modification that occurs in the particle modifier 360 maybe any modification to the crystalline structure or morphology of theparticles 112. For example, the particles 112 may be annealed in theparticle modifier 360 to densify the particles 112 or to recrystallizethe particles 112 into a polycrystalline or single crystalline form.Also, especially in the case of composite particles 112, the particlesmay be annealed for a sufficient time to permit redistribution withinthe particles 112 of different material phases. Particularly preferredparameters for such processes are discussed in more detail below.

The initial morphology of composite particles made in the furnace 110,according to the present invention, could take a variety of forms,depending upon the specified materials involved and the specificprocessing conditions. Examples of some possible composite particlemorphologies, manufacturable according to the present invention areshown in FIG. 47. These morphologies could be of the particles asinitially produced in the furnace 110 or that result from structuralmodification in the particle modifier 360. Furthermore, the compositeparticles could include a mixture of the morphological attributes shownin FIG. 47.

Referring now to FIG. 48, an embodiment of the apparatus of the presentinvention is shown that includes the aerosol generator 106 (in the formof the 400 transducer array design), the aerosol concentrator 236 (inthe form of a virtual impactor), the droplet classifier 280 (in the formof an impactor), the furnace 110, the particle cooler 320 (in the formof a gas quench cooler) and the particle collector 114 (in the form of abag filter). All process equipment components are connected viaappropriate flow conduits that are substantially free of sharp edgesthat could detrimentally cause liquid accumulations in the apparatus.Also, it should be noted that flex connectors 370 are used upstream anddownstream of the aerosol concentrator 236 and the droplet classifier280. By using the flex connectors 370, it is possible to vary the angleof slant of vertically extending slits in the aerosol concentrator 236and/or the droplet classifier 280. In this way, a desired slant for thevertically extending slits may be set to optimize the drainingcharacteristics off the vertically extending slits.

Aerosol generation with the process of the present invention has thusfar been described with respect to the ultrasonic aerosol generator. Useof the ultrasonic generator is preferred for the process of the presentinvention because of the extremely high quality and dense aerosolgenerated. In some instances, however, the aerosol generation for theprocess of the present invention may have a different design dependingupon the specific application. For example, when larger particles aredesired, such as those having a weight average size of larger than about3 μm, a spray nozzle atomizer may be preferred. For smaller-particleapplications, however, and particularly for those applications toproduce particles smaller than about 3 μm, and preferably smaller thanabout 2 μm in size, as is generally desired with the particles of thepresent invention, the ultrasonic generator, as described herein, isparticularly preferred. In that regard, the ultrasonic generator of thepresent invention is particularly preferred for when making particleswith a weight average size of from about 0.2 μm to about 3 μm.

Although ultrasonic aerosol generators have been used for medicalapplications and home humidifiers, use of ultrasonic generators forspray pyrolysis particle manufacture has largely been confined tosmall-scale, experimental situations. The ultrasonic aerosol generatorof the present invention described with reference to FIGS. 5–24,however, is well suited for commercial production of high qualitypowders with a small average size and a narrow size distribution. Inthat regard, the aerosol generator produces a high quality aerosol, withheavy droplet loading and at a high rate of production. Such acombination of small droplet size, narrow size distribution, heavydroplet loading, and high production rate provide significant advantagesover existing aerosol generators that usually suffer from at least oneof inadequately narrow size distribution, undesirably low dropletloading, or unacceptably low production rate.

Through the careful and controlled design of the ultrasonic generator ofthe present invention, an aerosol may be produced typically havinggreater than about 70 weight percent (and preferably greater than about80 weight percent) of droplets in the size range of from about 1 μm toabout 10 μm, preferably in a size range of from about 1 μm to about 5 μmand more preferably from about 2 μm to about 4 μm. Also, the ultrasonicgenerator of the present invention is capable of delivering high outputrates of liquid feed in the aerosol. The rate of liquid feed, at thehigh liquid loadings previously described, is preferably greater thanabout 25 milliliters per hour per transducer, more preferably greaterthan about 37.5 milliliters per hour per transducer, even morepreferably greater than about 50 milliliters per hour per transducer andmost preferably greater than about 100 millimeters per hour pertransducer. This high level of performance is desirable for commercialoperations and is accomplished with the present invention with arelatively simple design including a single precursor bath over an arrayof ultrasonic transducers. The ultrasonic generator is made for highaerosol production rates at a high droplet loading, and with a narrowsize distribution of droplets. The generator preferably produces anaerosol at a rate of greater than about 0.5 liter per hour of droplets,more preferably greater than about 2 liters per hour of droplets, stillmore preferably greater than about 5 liters per hour of droplets, evenmore preferably greater than about 10 liters per hour of droplets andmost preferably greater than about 40 liters per hour of droplets. Forexample, when the aerosol generator has a 400 transducer design, asdescribed with reference to FIGS. 7–24, the aerosol generator is capableof producing a high quality aerosol having high droplet loading aspreviously described, at a total production rate of preferably greaterthan about 10 liters per hour of liquid feed, more preferably greaterthan about 15 liters per hour of liquid feed, even more preferablygreater than about 20 liters per hour of liquid feed and most preferablygreater than about 40 liters per hour of liquid feed.

Under most operating conditions, when using such an aerosol generator,total particulate product produced is preferably greater than about 0.5gram per hour per transducer, more preferably greater than about 0.75gram per hour per transducer, even more preferably greater than about1.0 gram per hour per transducer and most preferably greater than about2.0 grams per hour per transducer.

One significant aspect of the process of the present invention formanufacturing particulate materials is the unique flow characteristicsencountered in the furnace relative to laboratory scale systems. Themaximum Reynolds number attained for flow in the furnace 110 with thepresent invention is very high, typically in excess of 500, preferablyin excess of 1,000 and more preferably in excess of 2,000. In mostinstances, however, the maximum Reynolds number for flow in the furnacewill not exceed 10,000, and preferably will not exceed 5,000. This issignificantly different from lab-scale systems where the Reynolds numberfor flow in a reactor is typically lower than 50 and rarely exceeds 100.

The Reynolds number is a dimensionless quantity characterizing flow of afluid which, for flow through a circular cross sectional conduit isdefined as:

${Re} = \frac{v\; d}{\mu}$where:

-   -   =fluid density;    -   v=fluid mean velocity;    -   d=conduit inside diameter; and    -   μ=fluid viscosity.        It should be noted that the values for density, velocity and        viscosity will vary along the length of the furnace 110. The        maximum Reynolds number in the furnace 110 is typically attained        when the average stream temperature is at a maximum, because the        gas velocity is at a very high value due to gas expansion when        heated.

One problem with operating under flow conditions at a high Reynoldsnumber is that undesirable volatilization of components is much morelikely to occur than in systems having flow characteristics as found inlaboratory-scale systems. The volatilization problem occurs with thepresent invention, because the furnace is typically operated over asubstantial section of the heating zone in a constant wall heat fluxmode, due to limitations in heat transfer capability. This issignificantly different than operation of a furnace at a laboratoryscale, which typically involves operation of most of the heating zone ofthe furnace in a uniform wall temperature mode, because the heating loadis sufficiently small that the system is not heat transfer limited.

With the present invention, it is typically preferred to heat theaerosol stream in the heating zone of the furnace as quickly as possibleto the desired temperature range for particle manufacture. Because offlow characteristics in the furnace and heat transfer limitations,during rapid heating of the aerosol the wall temperature of the furnacecan significantly exceed the maximum average target temperature for thestream. This is a problem because, even though the average streamtemperature may be within the range desired, the wall temperature maybecome so hot that components in the vicinity of the wall are subjectedto temperatures high enough to undesirably volatilize the components.This volatilization near the wall of the furnace can cause formation ofsignificant quantities of ultrafine particles that are outside of thesize range desired.

Therefore, with the present invention, it is preferred that when theflow characteristics in the furnace are such that the Reynolds numberthrough any part of the furnace exceeds 500, more preferably exceeds1,000, and most preferably exceeds 2,000, the maximum wall temperaturein the furnace should be kept at a temperature that is below thetemperature at which a desired component of the final particles wouldexert a vapor pressure not exceeding about 200 millitorr, morepreferably not exceeding about 100 millitorr, and most preferably notexceeding about 50 millitorr. Furthermore, the maximum wall temperaturein the furnace should also be kept below a temperature at which anintermediate component, from which a final component is to be at leastpartially derived, should also have a vapor pressure not exceeding themagnitudes noted for components of the final product.

In addition to maintaining the furnace wall temperature below a levelthat could create volatilization problems, it is also important thatthis not be accomplished at the expense of the desired average streamtemperature. The maximum average stream temperature must be maintainedat a high enough level so that the particles will have a desired highdensity. The maximum average stream temperature should, however,generally be a temperature at which a component in the final particles,or an intermediate component from which a component in the finalparticles is at least partially derived, would exert a vapor pressurenot exceeding about 100 millitorr, preferably not exceeding about 50millitorr, and most preferably not exceeding about 25 millitorr.

So long as the maximum wall temperature and the average streamtemperature are kept below the point at which detrimental volatilizationoccurs, it is generally desirable to heat the stream as fast as possibleand to remove resulting particles from the furnace immediately after themaximum stream temperature is reached in the furnace. With the presentinvention, the average residence time in the heating zone of the furnacemay typically be maintained at shorter than about 4 seconds, preferablyshorter than about 2 seconds, more preferably shorter than about 1second, still more preferably shorter than about 0.5 second, and mostpreferably shorter than about 0.2 second.

Another significant issue with respect to operating the process of thepresent invention, which includes high aerosol flow rates, is losswithin the system of materials intended for incorporation into the finalparticulate product. Material losses in the system can be quite high ifthe system is not properly operated if system losses are too high, theprocess would not be practical for use in the manufacture of particulateproducts of many materials. This has typically not been a majorconsideration with laboratory-scale systems.

One significant potential for loss with the process of the presentinvention is thermophoretic losses that occur when a hot aerosol streamis in the presence of a cooler surface. In that regard, the use of thequench cooler, as previously described, with the process of the presentinvention provides an efficient way to cool the particles withoutunreasonably high thermophoretic losses. There is also, however,significant potential for losses occurring near the end of the furnaceand between the furnace and the cooling unit.

It has been found that thermophoretic losses in the back end of thefurnace can be significantly controlled if the heating zone of thefurnace is operated such that the maximum stream temperature is notattained until near the end of the heating zone in the furnace, and atleast not until the last third of the heating zone. When the heatingzone includes a plurality of heating sections, the maximum averagestream temperature should ordinarily not occur until at least the lastheating section. Furthermore, the heating zone should typically extendto as close to the exit of the furnace as possible. This is counter toconventional thought which is to typically maintain the exit portion ofthe furnace at a low temperature to avoid having to seal the furnaceoutlet at a high temperature. Such cooling of the exit portion of thefurnace, however, significantly promotes thermophoretic losses.Furthermore, the potential for operating problems that could result inthermophoretic losses at the back end of the furnace are reduced withthe very short residence times in the furnace for the present invention,as discussed previously.

Typically, it would be desirable to instantaneously cool the aerosolupon exiting the furnace. This is not possible. It is possible, however,to make the residence time between the furnace outlet and the coolingunit as short as possible. Furthermore, it is desirable to insulate theaerosol conduit occurring between the furnace exit and the cooling unitentrance. Even more preferred is to insulate that conduit and, even morepreferably, to also heat that conduit so that the wall temperature ofthat conduit is at least as high as the average stream temperature ofthe aerosol stream. Furthermore, it is desirable that the cooling unitoperate in a manner such that the aerosol is quickly cooled in a mannerto prevent thermophoretic losses during cooling. The quench cooler,described previously, is very effective for cooling with low losses.Furthermore, to keep the potential for thermophoretic losses very low,it is preferred that the residence time of the aerosol stream betweenattaining the maximum stream temperature in the furnace and a point atwhich the aerosol has been cooled to an average stream temperature belowabout 200° C. is shorter than about 2 seconds, more preferably shorterthan about 1 second, and even more preferably shorter than about 0.5second and most preferably shorter than about 0.1 second. In mostinstances, the maximum average stream temperature attained in thefurnace will be greater than about 800° C. Furthermore, the totalresidence time from the beginning of the heating zone in the furnace toa point at which the average stream temperature is at a temperaturebelow about 200° C. should typically be shorter than about 5 seconds,preferably shorter than about 3 seconds, more preferably shorter thanabout 2 seconds, and most preferably shorter than about 1 second.

Another part of the process with significant potential forthermophoretic losses is after particle cooling until the particles arefinally collected. Proper particle collection is very important toreducing losses within the system. The potential for thermophoreticlosses is significant following particle cooling because the aerosolstream is still at an elevated temperature to prevent detrimentalcondensation of water in the aerosol stream. Therefore, cooler surfacesof particle collection equipment can result in significantthermophoretic losses.

To reduce the potential for thermophoretic losses before the particlesare finally collected, it is important that the transition between thecooling unit and particle collection be as short as possible.Preferably, the output from the quench cooler is immediately sent to aparticle separator, such as a filter unit or a cyclone. In that regard,the total residence time of the aerosol between attaining the maximumaverage stream temperature in the furnace and the final collection ofthe particles is preferably shorter than about 2 seconds, morepreferably shorter than about 1 second, still more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.Furthermore, the residence time between the beginning of the heatingzone in the furnace and final collection of the particles is preferablyshorter than about 6 seconds, more preferably shorter than about 3seconds, even more preferably shorter than about 2 seconds, and mostpreferably shorter than about 1 second. Furthermore, the potential forthermophoretic losses may further be reduced by insulating the conduitsection between the cooling unit and the particle collector and, evenmore preferably, by also insulating around the filter, when a filter isused for particle collection. The potential for losses may be reducedeven further by heating of the conduit section between the cooling unitand the particle collection equipment, so that the internal equipmentsurfaces are at least slightly warmer than the aerosol stream averagestream temperature. Furthermore, when a filter is used for particlecollection, the filter could be heated. For example, insulation could bewrapped around a filter unit, with electric heating inside of theinsulating layer to maintain the walls of the filter unit at a desiredelevated temperature higher than the temperature of filter elements inthe filter unit, thereby reducing thermophoretic particle losses towalls of the filter unit.

Even with careful operation to reduce thermophoretic losses, some losseswill still occur. For example, some particles will inevitably be lost towalls of particle collection equipment, such as the walls of a cycloneor filter housing. One way to reduce these losses, and correspondinglyincrease product yield, is to periodically wash the interior of theparticle collection equipment to remove particles adhering to the sides.In most cases, the wash fluid will be water, unless water would have adetrimental effect on one of the components of the particles. Forexample, the particle collection equipment could include parallelcollection paths. One path could be used for active particle collectionwhile the other is being washed. The wash could include an automatic ormanual flush without disconnecting the equipment. Alternatively, theequipment to be washed could be disconnected to permit access to theinterior of the equipment for a thorough wash. As an alternative tohaving parallel collection paths, the process could simply be shut downoccasionally to permit disconnection of the equipment for washing. Theremoved equipment could be replaced with a clean piece of equipment andthe process could then be resumed while the disconnected equipment isbeing washed.

For example, a cyclone or filter unit could periodically be disconnectedand particles adhering to interior walls could be removed by a waterwash. The particles could then be dried in a low temperature dryer,typically at a temperature of lower than about 50° C.

In one embodiment, wash fluid used to wash particles from the interiorwalls of particle collection equipment includes a surfactant. Some ofthe surfactant will adhere to the surface of the particles. This couldbe advantageous to reduce agglomeration tendency of the particles and toenhance dispersibility of the particles in a thick film pastformulation. The surfactant could be selected for compatibility with thespecific paste formulation anticipated.

Another area for potential losses in the system, and for the occurrenceof potential operating problems, is between the outlet of the aerosolgenerator and the inlet of the furnace. Losses here are not due tothermophoresis, but rather to liquid coming out of the aerosol andimpinging and collecting on conduit and equipment surfaces. Althoughthis loss is undesirable from a material yield standpoint, the loss maybe even more detrimental to other aspects of the process. For example,water collecting on surfaces may release large droplets that can lead tolarge particles that detrimentally contaminate the particulate product.Furthermore, if accumulated liquid reaches the furnace, the liquid cancause excessive temperature gradients within the furnace tube, which cancause furnace tube failure, especially for ceramic tubes.

One way to reduce the potential for undesirable liquid buildup in thesystem is to provide adequate drains, as previously described. In thatregard, it is preferred that a drain be placed as close as possible tothe furnace inlet to prevent liquid accumulations from reaching thefurnace. The drain should be placed, however, far enough in advance ofthe furnace inlet such that the stream temperature is lower than about80° C. at the drain location.

Another way to reduce the potential for undesirable liquid buildup isfor the conduit between the aerosol generator outlet and the furnaceinlet be of a substantially constant cross sectional area andconfiguration. Preferably, the conduit beginning with the aerosolgenerator outlet, passing through the furnace and continuing to at leastthe cooling unit inlet is of a substantially constant cross sectionalarea and geometry.

Another way to reduce the potential for undesirable buildup is to heatat least a portion, and preferably the entire length, of the conduitbetween the aerosol generator and the inlet to the furnace. For example,the conduit could be wrapped with a heating tape to maintain the insidewalls of the conduit at a temperature higher than the temperature of theaerosol. The aerosol would then tend to concentrate toward the center ofthe conduit due to thermophoresis. Fewer aerosol droplets would,therefore, be likely to impinge on conduit walls or other surfacesmaking the transition to the furnace.

Another way to reduce the potential for undesirable liquid buildup is tointroduce a dry gas into the aerosol between the aerosol generator andthe furnace. Referring now to FIG. 49, one embodiment of the process isshown for adding a dry gas 118 to the aerosol 108 before the furnace110. Addition of the dry gas 118 causes vaporization of at least a partof the moisture in the aerosol 108, and preferably substantially all ofthe moisture in the aerosol 108, to form a dried aerosol 119, which isthen introduced into the furnace 110.

The dry gas 118 will most often be dry air, although in some instancesit may be desirable to use dry nitrogen gas or some other dry gas. Ifsufficient a sufficient quantity of the dry gas 118 is used, thedroplets of the aerosol 108 are substantially completely dried tobeneficially form dried precursor particles in aerosol form forintroduction into the furnace 110, where the precursor particles arethen pyrolyzed to make a desired particulate product. Also, the use ofthe dry gas 118 typically will reduce the potential for contact betweendroplets of the aerosol and the conduit wall, especially in the criticalarea in the vicinity of the inlet to the furnace 110. In that regard, apreferred method for introducing the dry gas 118 into the aerosol 108 isfrom a radial direction into the aerosol 108. For example, equipment ofsubstantially the same design as the quench cooler, described previouslywith reference to FIGS. 41–43, could be used, with the aerosol 108flowing through the interior flow path of the apparatus and the dry gas118 being introduced through perforated wall of the perforated conduit.An alternative to using the dry gas 118 to dry the aerosol 108 would beto use a low temperature thermal preheater/dryer prior to the furnace110 to dry the aerosol 108 prior to introduction into the furnace 110.This alternative is not, however, preferred.

Still another way to reduce the potential for losses due to liquidaccumulation is to operate the process with equipment configurationssuch that the aerosol stream flows in a vertical direction from theaerosol generator to and through the furnace. For smaller-sizeparticles, those smaller than about 1.5 μm, this vertical flow should,preferably, be vertically upward. For larger-size particles, such asthose larger than about 1.5 μm, the vertical flow is preferablyvertically downward.

Furthermore, with the process of the present invention, the potentialfor system losses is significantly reduced because the total systemretention time from the outlet of the generator until collection of theparticles is typically shorter than about 10 seconds, preferably shorterthan about 7 seconds, more preferably shorter than about 5 seconds andmost preferably shorter than about 3 seconds.

For the production of nickel particles according to the presentinvention, the liquid feed 102 includes at least one nickel precursorfor preparation of the nickel particles 112. The nickel precursor may bea substance in either a liquid or solid phase of the liquid feed 102.Typically, the nickel precursor will be a nickel-containing compound,such as a salt, dissolved in a liquid solvent of the liquid feed 102.The nickel precursor may undergo one or more chemical reactions in thefurnace 110 to assist in production of the nickel particles 112.Alternatively, the nickel precursor may contribute to formation of thenickel particles 112 without undergoing chemical reaction. This could bethe case, for example, when the liquid feed 102 includes suspendedparticles as a precursor material.

The liquid feed 102 thus includes the chemical components that will formthe nickel particles 112. For example, the liquid feed 102 can comprisea solution containing nitrates, chlorides, sulfates, hydroxides, oroxalates of nickel. A preferred precursor to nickel according to thepresent invention is nickel nitrate, Ni(NO₃)₂. Nickel nitrate is highlysoluble in water and the solutions maintain a low viscosity, even athigh concentrations. The solution preferably has a nickel precursorconcentration that is unsaturated to avoid precipitate formation in theliquid. The solution preferably includes a soluble precursor to yield aconcentration of from about 1 to about 50 weight percent nickel, morepreferably from about 2.5 to about 15 weight percent nickel, such asfrom about 2.5 to 7.5 weight percent nickel, particularly for theformation of particles having an average size of from about 0.3 μm to0.8 μm. The final particle size of the nickel particles 112 isinfluenced by the precursor concentration. Generally, lower precursorconcentrations in the liquid feed will produce particles having asmaller average particle size.

Preferably, the solvent is aqueous-based for ease of operation, althoughother solvents, such as toluene, may be desirable. The use of organicsolvents can lead to carbon in the metal particles. The pH of theaqueous-based solutions can be adjusted to alter the solubilitycharacteristics of the precursor in the solution.

The precursor solution can also include other additives. For example, areducing agent to facilitate the reaction of the precursor to a metalparticle can advantageously be included in the precursor solution. Theuse of a reducing agent in the solution may eliminate or reduce the needfor a reducing gas, such as hydrogen, in the carrier gas. A preferredreducing agent according to the present invention is hydrazine (H₂NNH₂),which can be included in the precursor solution in an amount of, forexample, from about 1 to about 15 weight percent. Other reducing agents,such as borohydrides (MBR_(4-x)H_(x), where x=1 to 4, R is an alkyl oraryl, such as methyl or ethyl, and M is Li, Na, K, or NH₄) may also beuseful.

In addition to the foregoing, the liquid feed 102 may also include otheradditives that contribute to the formation of the particles. Forexample, a fluxing agent can be added to this solution to increase thecrystallinity and/or density of the particles. For example, the additionof urea to metal salt solutions, such as nickel nitrate, can increasethe density of particles produced from the solution. In one embodiment,up to about 1 mole equivalent urea is added to the precursor solution.Further, if the particles are to be coated nickel particles, as isdiscussed in more detail below, soluble precursors to both the nickelparticle and the coating can be used in the precursor solution whereinthe coating precursor is an involatile or volatile species.

Thus, the liquid feed 102 may include multiple precursor materials,which may be present together in a single phase or separately inmultiple phases. The liquid feed 102 may include multiple precursors insolution in a single liquid vehicle. For example, the liquid feed cancomprise a solution of two different metal precursors adapted to form aparticle 112 that is a metal alloy or intermetallic compound.Alternatively, one precursor material could be in a solid particulatephase and a second precursor material could be in a liquid phase. Also,one precursor material could be in one liquid phase and a secondprecursor material could be in a second liquid phase, such as when theliquid feed 102 comprises an emulsion. When the liquid feed 102 includesmultiple precursors, more than one of the precursors may contain nickel,or one or more of the precursors may contain a component other thannickel that is contributed to the particles 112. Different componentscontributed by different precursors may be present in the particlestogether in a single material phase, or the different components may bepresent in different material phases when the nickel particles 112 arecomposites of multiple phases.

To form metal composite particles, the liquid feed can include colloids,for example boehmite particles or silica particles. Particles as largeas about 0.3 μm can be suspended in the aerosol droplets using anultrasonic nebulizer. The suspended colloids can also coat the outersurface of the metal particles (forming a particulate coating),depending on the process conditions and the selected materials. Theparticles can also be formed such that a metal phase uniformly coats acore of a non-metallic phase.

A carrier gas 104 under controlled pressure is introduced to the aerosolgenerator to move the droplets away from the generator. The carrier gas104 may comprise any gaseous medium in which droplets produced from theliquid feed 102 may be dispersed in aerosol form. Also, the carrier gas104 may be inert, in that the carrier gas 104 does not participate inthe formation of the nickel particles 12. Alternatively, the carrier gas104 may have one or more active component(s), such as hydrogen gas, thatcontribute to formation of the nickel particles 112. In that regard, thecarrier gas may include one or more reactive components that react inthe furnace 110 to contribute to formation of the nickel particles 112.Examples of preferred carrier gases include reactive carrier gases suchas air or oxygen and inert carrier gases such as argon or nitrogen.Reducing gas compositions, such as those including hydrogen (H₂), canalso be used to produce nickel metal particles.

According to the present invention for the production of nickelparticles, particularly nickel metal particles, it is preferred to use acarrier gas including hydrogen as the forming gas. Preferably, thehydrogen content of the carrier gas is at least about 2 volume percent,more preferably is at least about 2.5 volume percent and even morepreferably at least about 3 volume percent. The hydrogen gas can bemixed with, for example, nitrogen gas. Reduced levels of hydrogen gascan be used if the precursor solution includes a reducing agent, as isdiscussed above.

To form substantially uniform coatings on the surface of the nickelmetal particles such as those discussed above, a reactive gascomposition can be contacted with the nickel metal particle at anelevated temperature after the particle has been formed. For example,the reactive gas can be introduced into the heated reaction chamber atthe distal end so that the desired compound deposits on the surface ofthe particle.

More specifically, the droplets can enter the heated reaction zone at afirst end such that the precursor droplets move through the heated zoneand form the nickel particles. At the opposite end of the heatedreaction zone, a reactive gas composition can be introduced such thatthe reactive gas composition contacts the nickel particles at anelevated temperature. Alternatively, the reactive gas composition can becontacted with the heated particles in a separate heating zone locateddownstream from the heated reaction zone.

For example, precursors to metal oxide coatings can be selected fromvolatile metal acetates, chlorides, alkoxides or halides. Suchprecursors are known to react at high temperatures to form thecorresponding metal oxides and eliminate supporting ligands or ions. Forexample, SiCl₄ can be used as a precursor to SiO₂ coatings when watervapor is present:SiCl_(4(g))+2H₂O_((g))------->SiO_(2(s))+4HCl_((g))SiCl₄ also is highly volatile and is a liquid at room temperature, whichmakes transport into the reactor more controllable.

Metal alkoxides can be used to produce metal oxide films by hydrolysis.The water molecules react with the alkoxide M—O bond resulting in cleanelimination of the corresponding alcohol with the formation of M—O—Mbonds:Si(OEt)₄+2H₂O------->SiO₂+4EtOHMost metal alkoxides have a reasonably high vapor pressure and aretherefore well suited as coating precursors.

Metal acetates are also useful as coating precursors since they readilydecompose upon thermal activation by acetic anhydride elimination:Mg(O₂CCH₃)₂------->MgO+CH₃C(O)OC(O)CH₃Metal acetates are advantageous as coating precursors since they arewater stable and are reasonably inexpensive.

Coatings can be generated on the particle surface by a number ofdifferent mechanisms. One or more precursors can vaporize and fuse tothe hot particle surface and thermally react resulting in the formationof a thin-film coating by chemical vapor deposition (CVD). Preferredcoatings deposited by CVD include metal oxides and elemental metals.Further, the coating can be formed by physical vapor deposition (PVD)wherein a coating material physically deposits on the surface of theparticles. Preferred coatings deposited by PVD include organic materialsand elemental metals. Alternatively, the gaseous precursor can react inthe gas phase forming small particles, for example less than about 5nanometers in size, which then diffuse to the larger particle surfaceand sinter onto the surface, thus forming a coating. This method isreferred to as gas-to-particle conversion (GPC). Whether such coatingreactions occur by CVD, PVD or GPC is dependent on the reactorconditions, such as temperature, precursor partial pressure, waterpartial pressure and the concentration of particles in the gas stream.Another possible surface coating method is surface conversion of thesurface of the particles by reaction with a vapor phase reactant toconvert the surface of the particles to a different material than thatoriginally contained in the particles.

In addition, a volatile coating material such as PbO, MoO₃ or V₂O₅ canbe introduced into the reactor such that the coating deposits on theparticles by condensation. Highly volatile metals, such as silver, canalso be deposited by condensation. Further, the particles can be coatedusing other techniques. For example, a soluble precursor to both thenickel powder and the coating can be used in the precursor solutionwherein the coating precursor is involatile, (e.g. Al(NO₃)₃ or volatile(e.g. Sn(OAc)₄ where Ac is acetate). In another embodiment, a colloidalprecursor and a soluble nickel precursor can be used to form aparticulate colloidal coating on the nickel particle. It will beappreciated that multiple coatings can be deposited on the surface ofthe metal particles if such multiple coatings are desirable.

The coatings are preferably as thin as possible while maintainingconformity about particle such that the metal is not substantiallyexposed. For example, the coatings can have an average thickness of notgreater than about 200 nanometers, preferably not greater than about 100nanometers, and more preferably not greater than about 50 nanometers.For most applications, the coating should have an average thickness ofat least about 5 nanometers.

The present invention is directed to nickel powder batches includingnickel particles wherein the nickel particles constituting the powderbatch preferably have a spherical morphology, a small average particlesize and a narrow particle size distribution. The powders of the presentinvention offer numerous advantages over conventional nickel powders andare particularly useful in a number of applications including thefabrication of microelectronic devices, where the powders are dispersedin thick film pastes used to form electrically conductive layers orpaths in devices such as multilayer capacitors and multi-chip modules.Similar pastes are also useful in other applications, such as for theformation of electrodes in flat panel display devices.

The nickel powder batches according to the present invention include acommercially useful quantity of nickel particles. The nickel particlesinclude nickel in the form of a metal or a nickel compound such asnickel boride or nickel oxide. According to one embodiment, the nickelparticles include a metal phase having at least about 50 weight percentnickel metal, and depending upon the application, the particlespreferably include at least about 80 weight percent nickel metal andeven more preferably at least about 90 weight percent nickel metal.

For many applications, the nickel particles can be metal alloyparticles, such as nickel metal alloy particles wherein nickel metal isalloyed with one or more alloying elements including, but not limitedto, palladium (Pd), silver (Ag), gold (Au), copper (Cu), tungsten (W),molybdenum (Mo), platinum (Pt), iron (Fe), tin (Sn) and cobalt (Co). Inone preferred embodiment, the alloying element is palladium. As usedherein, the term metal alloy particles include intermetallic compoundsthat form between nickel and other metals, such as those that commonlyform between nickel and aluminum.

The metal alloy particles according to this embodiment of the inventionare preferably homogenous, well-mixed on the atomic level, and havesubstantially no phase segregation of the nickel metal and the othermetal element. However, it may be desirable for some applications thatthe particles consist of distinct metal phases that are segregated (seeFIG. 47 e). Depending upon the intended application, the other metalelement can preferably be included in an amount of from about 0.1 toabout 40 weight percent, such as from about 1 to about 15 weightpercent, based on the total amount of metal.

Such additional metal elements can modify the properties of the metalparticles in several ways, as compared to pure nickel metal particles.These modifications can include an increased or decreased sinteringtemperature, which is the temperature at which individual particlesbegin to coalesce due to softening and diffusion. The meltingtemperature can also be increased or decreased. The vaporization ofmetal at the synthesis temperature can be inhibited, which reduces theformation of ultrafine particles from the vapors. Ultrafine particlescan be detrimental to the dispersion properties of the powder. Further,the alloying element can improve the rheological properties of theparticles for better dispersion of the particles in organic andwater-based pastes. The oxidation resistance can be improved such as byincreasing the temperature at which oxidation begins or by reducing thetotal amount of metal that will oxidize at a given temperature andpartial pressure of oxygen. Adhesion of the metal with ceramics can alsobe improved by alloying the particles. The alloyed particles can also beuseful as a catalyst material.

The metal alloy particles can be formed in accordance with themethodology described above. Typically, the particle will be formed froma liquid solution which includes both a nickel metal precursor and aprecursor for the additional element. The concentration level can easilybe adjusted by adjusting the relative ratios of nickel metal precursorand other metal precursor(s) in the liquid solution. For example,nickel/palladium alloy particles can be formed from a solution of nickelnitrate and palladium nitrate.

The nickel powder batches produced according to the present inventioninclude particles having a small average particle size. Although thepreferred average size of the particles will vary according to theparticular application of the powder, the weight average particle sizeof the particles is preferably at least about 0.1 μm and is preferablynot greater than about 5 μm. For most applications, the weight averageparticle size is more preferably not greater than about 3 μm and evenmore preferably is not greater than about 2 μm, such as from about 0.3μm to about 1.5 μm.

A particularly preferred weight average particle size for the nickelpowder batches according to the present invention is from about 0.3 μmto about 0.8 μm. Nickel powders having such an average particle size areparticularly useful in microelectronic applications wherein conductivemetal powders are dispersed in a thick film paste which is applied to asubstrate and heated to form a nickel metal film or line. Utilizingnickel metal powder having such a small average particle size enablesthe formation of conductive traces having a narrower width and filmshaving a decreased thickness. Such powder batches are particularlyuseful for the internal electrodes of multilayer ceramic capacitors,which require a thin and uniform, defect-free conductive film.

According to a preferred embodiment of the present invention, the powderbatch of nickel particles has a narrow particle size distribution, suchthat the majority of particles are about the same size. Preferably, atleast about 90 weight percent and more preferably at least about 95weight percent of the particles have a size that is not larger thantwice the weight average particle size. For example, when the averageparticle size is about 1 μm, it is preferred that at least about 90weight percent of the particles are not larger than 2 μm and it is morepreferred that at least about 95 weight percent of the particles are notlarger than 2 μm. Further, it is preferred that at least about 90 weightpercent and more preferably at least about 95 weight percent of theparticles have a size that is not larger than about 1.5 times the weightaverage particle size. For example, when the average particle size isabout 1 μm, it is preferred that at least about 90 weight percent of theparticles are not larger than 1.5 μm and it is more preferred that atleast about 95 weight percent of the particles are not larger than 1.5μm.

It is also possible according to the present invention to provide anickel powder batch having a bimodal particle size distribution. Thatis, the powder batch can include nickel particles having two distinctand different average particle sizes, each with a narrow sizedistribution as discussed above. Such bimodal distributions can enhancethe packing efficiency of the powder in a variety of applications.

The nickel particles of the present invention can be substantiallysingle crystal particles or may be comprised of a number ofcrystallites. Nickel metal particles having a high crystallinity, i.e.large average crystallite size, enhance the electrical properties ofdevices formed from the powder. Highly crystalline particles will alsoincrease the oxidation resistance of the powder by reducing the ratio ofcrystallite surface area to total particle volume.

According to one embodiment of the present invention, it is preferredthat the average crystallite size is close to the average particle sizesuch that the particles are mostly single crystals or are composed ofonly a few large crystals. Accordingly, the average crystallite size ispreferably at least about 40 nanometers, more preferably is at leastabout 60 nanometers, even more preferably is at least about 80nanometers, and most preferably is at least about 100 nanometers. In oneembodiment, the average crystallite size is at least about 200nanometers. As the average crystallite size relates to the averageparticle size disclosed above, the average crystallite size ispreferably at least about 20 percent of the average particle size, morepreferably is at least about 30 percent of the average particle size andeven more preferably is at least about 40 percent of the averageparticle size. Nickel metal particles having such high crystallinityadvantageously have enhanced electrical properties, including higherconductivity, and also improved oxidation resistance as compared tonickel metal powders having lower crystallinity, i.e., a smaller averagecrystallite size. As the average crystallite size approaches the averageparticle size, the nickel particles remain substantially spherical, butcan appear faceted on the outer surface of the particle.

The nickel particles produced according to the present invention alsohave a high degree of purity and it is preferred that the particlesinclude not greater than about 0.1 atomic percent impurities and morepreferably not greater than about 0.01 atomic percent impurities. Sinceno milling of the particles is required to achieve the small averageparticle sizes disclosed herein, there are substantially no undesiredimpurities such as alumina, zirconia or high carbon steel in the powderbatch.

The nickel particles according to the present invention are alsopreferably dense (e.g. not hollow or porous), as measured by heliumpycnometry. Preferably, the nickel particles according to the presentinvention have a particle density of at least about 80% of thetheoretical density (at least about 7.1 g/cm³ for pure nickel), morepreferably at least about 90% of the theoretical density (at least about8.0 g/cm³ for pure nickel) and even more preferably at least about 95%of the theoretical density (at least about 8.4 g/cm³ for pure nickel).In one embodiment, the particle density is at least about 99% of thetheoretical density. The theoretical density can be easily calculatedfor multi-phase compositions, including alloys and composites, basedupon the relative percentages of each component. High density particlesprovide many advantages over porous particles, including reducedshrinkage during sintering.

The nickel particles according to a preferred embodiment of the presentinvention are also substantially spherical in shape. That is, theparticles are not jagged or irregular in shape. Spherical particles areparticularly advantageous because they disperse more readily in a pasteor slurry and impart advantageous flow characteristics to pastecompositions. Although the particles are substantially spherical, theycan become faceted as the crystallite size increases and approaches theaverage particle size.

The nickel powder according to the present invention also has a lowsurface area. As is discussed above, the particles are substantiallyspherical, which reduces the total surface area for a given mass ofpowder. Further, the elimination of larger particles from the powderbatch eliminates the porosity that is associated with open pores on thesurface of such larger particles. Due to the substantial elimination ofthe larger particles with open porosity and the spherical shape of theparticles, the powder advantageously has a lower surface area. Surfacearea is typically measured using the BET nitrogen adsorption methodwhich is indicative of the gas-accessible surface area of the powder,including the surface area of accessible surface pores. For a givenparticle size distribution, a lower value of a surface area per unitmass of powder generally indicates solid and non-porous particles.According to one embodiment of the present invention, the nickel powderpreferably has a specific surface area of not greater than about 3 m²/g,more preferably not greater than about 2 m²/g. Decreased surface areareduces the susceptibility of the powders to adverse surface reactions,such as oxidation of the metal. This characteristic can advantageouslyextend the shelf-life of such powders.

The surfaces of the nickel particles according to the present inventionare typically smooth and clean and preferably have a minimal depositionof ultrafine particles (e.g., less than about 40 nanometers) on theparticle surface. It is believed that such ultrafine particles caninhibit the ability of the particles to adequately disperse in a thickfilm paste composition. Further, the surface of the nickel particles issubstantially free of surfactants or other organic contaminants. Metalparticles made by liquid precipitation routes are often contaminatedwith residual surfactants from the manufacturing process. Suchsurfactants can hinder the dispersibility of the metal powders in apaste.

The powder batches of nickel metal particles according to the presentinvention are preferably also substantially unagglomerated, that is,they include substantially no hard agglomerates of particles. Hardagglomerates are physically coalesced lumps of two or more particlesthat behave as one larger, irregularly shaped particle. Agglomerates aredisadvantageous in most applications. For example, when agglomeratedmetal powders are used in a thick film paste, the sintered film that isformed can contain lumps that lead to a defective product. Accordingly,it is preferred that no more than about 0.5 weight percent of the nickelparticles in the powder batch of the present invention are in the formof hard agglomerates and more preferably no more than about 0.1 weightpercent of the particles are in the form of hard agglomerates.

According to one embodiment of the present invention, the nickelparticles are metal composite particles, wherein the individualparticles include a metal phase and at least one non-metallic phaseassociated with the metal phase, such as one that is dispersedthroughout the metal phase. For example, the metal composite particlescan include a metal oxide dispersed throughout a nickel metal phase.Preferred simple metal oxides can include, but are not limited to, NiO,SiO₂, Cu₂O, CuO, B₂O₃, TiO₂, ZrO₂, Bi₂O₃, CaO, V₂O₅, and Al₂O₃. Also,the metal composite particles can include a metal phase and anon-metallic phase comprising carbon. Such a composite can be formed bydispersing a particulate carbon precursor in a nickel precursor andforming the particles as described hereinabove.

Metal oxides can advantageously modify the sintering characteristics orother properties of the nickel metal particles, such as by increasingthe sintering temperature of the powder, modifying the thermal expansioncharacteristics of the powder or improving the adhesion of the metal toa substrate. Further, oxides can be used as an inexpensive fillermaterial, reducing the volume of the nickel metal that is used withoutsubstantially reducing conductivity. Metal oxides can also increase theoxidation resistance of the metal particles.

Depending upon the application of the nickel metal powder batch, thecomposite particles preferably include at least about 0.1 weight percentof the non-metallic phase and more preferably from about 0.2 to about 50weight percent of the non-metallic phase, and even more preferably fromabout 0.2 to about 35 weight percent of the non-metallic phase. For someapplications, such as in MLCC capacitors, it is preferred to incorporatefrom about 0.2 to about 5 weight percent of the non-metallic phase, suchas from about 0.5 to about 2 weight percent. More than one non-metallicsecond phase can also be included in the particles. The morphology anddistribution of the metal and non-metallic phases can vary, but it ispreferred that the non-metallic phase is homogeneously dispersedthroughout the metal phase.

For some applications, such as MLCC's discussed in more detailhereinbelow, it is advantageous to provide metal composite particlesincluding a metal phase and a non-metallic phase of a ceramic dielectriccompound, preferably from about 0.5 to about 2 weight percent of adielectric compound. Such a composite particle is particularly usefulfor the internal electrodes of an MLCC. Such metal composite powdersadvantageously provide improved adhesion between the ceramic dielectriclayers and the metal layers as well as improved thermal expansioncharacteristics during sintering of the MLCC. That is, the thermalexpansion characteristics of the powder will closely match that of thedielectric. This property will advantageously result in fewer rejectionsof the devices due to delamination, cracks or camber.

Preferred dielectric compounds for incorporation into the nickel metalparticles include titanates, zirconates, silicates, aluminates, niobatesand tantalates. Particularly preferred dielectric compounds aretitanates such as barium titanate, neodymium titanate, magnesiumtitanate, calcium titanate, lead titanate and strontium titanate. Alsoparticularly preferred are zirconates such as magnesium zirconate orcalcium zirconate and niobate compounds, commonly referred to as relaxordielectrics. Those skilled in the art will recognize that manydielectric compounds are a combination of the foregoing and/or arenon-integral stoichiometry compounds, such as BaTi_(0.903)Zr_(0.097)O₃.

When the particles are to be used to form a conductive film on aceramic, it is often preferred to include a ceramic compound dispersedin the metal composite particles that is the same or has similar thermalexpansion characteristics as the ceramic used to form the ceramicsubstrate. For example, a particularly preferred embodiment for thefabrication of MLCC's utilizes barium titanate as the ceramic dielectriclayer and metal composite particles including a metal and bariumtitanate.

According to another embodiment of the present invention, the nickelparticles are coated particles that include a particulate coating (FIG.47 d) or non-particulate (film) coating (FIG. 47 a) that substantiallyencapsulates an outer surface of the particles. The coating can be ametal or can be a non-metallic compound, such as a boride. Preferably,the coating is very thin and uniform and has an average thickness of notgreater than about 200 nanometers, more preferably not greater thanabout 100 nanometers, and even more preferably not greater than about 50nanometers. While the coating is thin, the coating should substantiallyencapsulate the entire particle such that substantially none of theoriginal particle surface is exposed. Accordingly, the coatingpreferably has an average thickness of at least about 5 nanometers.

As is discussed above, the coating can be a metal, a metal oxide orother inorganic compound, or can be an organic compound. For example,nickel particles can be coated with a metal, such as a more costly noblemetal, to obtain the surface properties of the noble metal at a reducedcost. Thus, the nickel particles can be coated with platinum or gold toobtain an oxidation resistant powder at a reduced cost. According to onepreferred embodiment, the nickel particles are coated with silver or asilver alloy. According to another embodiment, the nickel particles arecoated with a copper-based metal such as copper or a copper alloy. Suchcopper-coated particles are particularly useful when the nickel metalparticles are used with a neodymium-based ceramic, such as neodymiumtitanate, in an MLCC. The copper metal advantageously inhibits leachingof the neodymium into the nickel metal.

Alternatively, a metal oxide coating can be selected, such as a metaloxide selected from the group consisting of ZrO₂, NiO, SiO₂, B₂O₅, TiO₂,Cu₂O, CuO, Bi₂O₃, V₂O₅ and Al₂O₃. Among these, SiO₂ and Al₂O₃ are oftenpreferred. Metal oxide coatings can advantageously inhibit the sinteringof nickel metal particles and can also improve the dispersibility of theparticles in a paste. The coatings can also increase the oxidationresistance of metal particles and increase the corrosion resistance in avariety of conditions. The nickel particles can include more than onecoating, if multiple coatings are desirable. In addition to theforegoing, the coating can comprise a ceramic dielectric compound, suchas those described above with reference to metal composite particles.

The nickel particles of the present invention can also be coated with anorganic compound, for example to provide improved dispersion which willresult in smoother prints having lower lump counts when applied as apaste. The organic coating can advantageously be placed onto apreviously formed metal oxide coating encapsulating the metal particle,as is discussed above. The organic compound for coating the particlescan be selected from a variety of organic compounds such as PMMA(polymethylmethacrylate), polystyrene or the like. The organic coatingcan also comprse a surfactant for improving the dispersibility of thepowders in a flowable medium, such as a paste. The organic coatingpreferably has an average thickness of not greater than about 100nanometers, more preferably not greater than about 50 nanometers and issubstantially dense and continuous about the particle. The organiccoatings can advantageously reduce corrosion of the particles and alsocan improve the dispersion characteristics of the particles in a pasteor slurry.

The coating can also be comprised of one or more monolayer coatings,such as from about 1 to 3 monolayer coating. A monolayer coating isformed by the reaction of an organic or an inorganic molecule with thesurface of the particles to form a coating layer that is essentially onemolecular layer thick. In particular, the formation of a monolayercoating by reaction of the surface of the particle with a functionalizedorgano silane such as halo- or amino-silanes, for examplehexamethyldisilazane or trimethylsilylchloride, can be used to modifythe hydrophobicity and hydrophilicity of the powders. Such coatingsallow for greater control over the dispersion characteristics of thepowder in a variety of thick film paste compositions.

The monolayer coatings may also be applied to nickel particles that havealready been coated with an organic or inorganic coating thus providingbetter control over the corrosion characteristics (through the use ofthicker coating) as well as dispersibility (through the monolayercoating) of the particles.

The nickel powder batches according to the present invention, includingpowder batches comprising composite particles and coated particles, areuseful in a number of applications and can be used to fabricate a numberof novel devices and intermediate products. Such devices andintermediate products are included within the scope of the presentinvention.

One preferred class of intermediate products according to the presentinvention are thick film paste compositions, also referred to as thickfilm inks. These pastes are particularly useful in the microelectronicsindustry for the application of conductors, resistors and dielectricsonto a substrate and in the flat panel display industry for applyingconductors, dielectrics and phosphors onto a panel.

In the thick film process, a viscous paste that includes a functionalparticulate phase (metals, dielectrics, metal oxides, etc . . . ) isscreen printed onto a substrate. A porous screen fabricated fromstainless steel, polyester, nylon or similar inert material is stretchedand attached to a rigid frame. A predetermined pattern is formed on thescreen corresponding to the pattern to be printed. For example, a UVsensitive emulsion can be applied to the screen and exposed through apositive or negative image of the design pattern. The screen is thendeveloped to remove portions of the emulsion in the pattern regions.

The screen is then affixed to a printing device and the thick film pasteis deposited on top of the screen. The substrate to be printed is thenpositioned beneath the screen and the paste is forced through the screenand onto the substrate by a squeegee that traverses the screen. Thus, apattern of traces and/or pads of the paste material is transferred tothe substrate. The substrate with the paste applied in a predeterminedpattern is then subjected to a drying and sintering treatment tosolidify and adhere the functional phase to the substrate.

Thick film pastes have a complex chemistry and generally include afunctional phase, a binder phase and an organic vehicle phase. Thefunctional phase includes the nickel powders of the present inventionwhich can provide conductivity for electrical transmission and areuseful in components such as capacitors. The particle size, sizedistribution, surface chemistry and particle shape of the nickelparticles all influence the rheology of the paste, as well as thecharacteristics of the sintered film.

The binder phase is typically a mixture of inorganic binders such asmetal oxide or glass frit powders. For example, PbO based glasses arecommonly used as binders. The function of the binder phase is to controlthe sintering of the film and assist the adhesion of the functionalphase to the substrate and/or assist in the sintering of the functionalphase. Reactive compounds can also be included in the paste to promoteadherence of the functional phase to the substrate. For example, 0.1 to1 percent CuO or CdO can be included in a metal paste applied to analumina substrate. The CuO or CdO reacts to form an aluminate whichprovides improved adhesion of the metal film.

Thick film pastes also include an organic vehicle phase that is amixture of solvents, polymers, resins or other organics whose primaryfunction is to provide the appropriate rheology (flow properties) to thepaste. The liquid solvent assists in mixing of the components into ahomogenous paste and substantially evaporates upon application of thepaste to the substrate. Usually the solvent is a volatile liquid such asmethanol, ethanol, terpineol, butyl carbitol, butyl carbitol acetate,aliphatic alcohols, esters, acetone and the like. The other organicvehicle components can include thickeners (sometimes referred to asorganic binders), stabilizing agents, surfactants, wetting agents andthe like. Thickeners provide sufficient viscosity to the paste and alsoacts as a binding agent in the unfired state. Examples of thickenersinclude ethyl cellulose, polyvinyl acetate, resins such as acrylicresin, cellulose resin, polyester, polyamide and the like. Thestabilizing agents reduce oxidation and degradation, stabilize theviscosity or buffer the pH of the paste. For example, triethanolamine isa common stabilizer. Wetting agents and surfactants are well known inthe thick film paste art and can include triethanolamine and phosphateesters. In addition to the foregoing, oxidation resistant additives canbe included in the paste to reduce oxidation of nickel metal. Forexample, the incorporation of boron-containing additives, includingborate glasses, is known to inhibit the oxidation of nickel.

The different components of the thick film paste are mixed in thedesired proportions in order to produce a substantially homogenous blendwherein the functional phase is well dispersed throughout the paste.Typically, the thick film paste will include from about 5 to about 95weight percent, such as from about 60 to 85 weight percent, of thefunctional phase, including the nickel powders of the present invention.

Some applications of thick film pastes require higher tolerances thancan be achieved using standard thick-film technology, as is describedabove. As a result, some thick film pastes have photo-imaging capabilityto enable the formation of lines and traces with decreased width andpitch. In this type of process, a photoactive thick film paste is laiddown substantially as is described above. The paste is then dried andexposed to ultraviolet light through a photomask and the exposedportions of the paste are developed to remove unwanted portions of thepaste. This technology permits higher density interconnections andconductive traces to be formed. The combination of the foregoingtechnology with the nickel powders of the present invention permits thefabrication of devices with increased circuit density and tolerances ascompared to conventional technologies using conventional powders.

Examples of thick-film pastes are disclosed in U.S. Pat. Nos. 4,172,733;3,803,708; 4,140,817; and 3,816,097 all of which are incorporated hereinby reference in their entirety.

The nickel metal powder of the present invention is particularlyadvantageous for many applications of thick film pastes. Nickel issignificantly less expensive than the noble metals that are used inthick film pastes for many applications. Nickel also has a linearthermal expansion coefficient of about 13.3 ppm/° C., which closelymatches that of ceramic substrate materials such as alumina or ceramicdielectrics. Nickel metal is also advantageous because nickel metalresists leaching when soldered. Some other metals can leachsignificantly when the metal is soldered. Nickel metal is alsoadvantageous because of its high conductivity.

One of the disadvantages of nickel metal that has limited its widespreaduse is that nickel must be sintered in a reducing atmosphere due to thestrong tendency of nickel to oxidize at relatively low temperatures(e.g., at about 600° C. or lower) in oxygen-containing atmospheres, suchas air. This presents a significant problem, particularly in themanufacture of multilayer ceramic capacitors that include an oxidedielectric (e.g., BaTiO₃) that must be sintered at high temperatures inan oxygen-containing atmosphere to avoid reduction of the oxide ceramic.The nickel metal powders of the present invention alleviate a number ofthese problems through the unique combination of high crystallinity andlow surface area.

As is discussed above, the nickel metal powders according to the presentinvention have a small average particle size, are substantiallyspherical in shape, have a narrow particle size distribution and have areduced number of ultrafine particles on the surface of the particles.Due to this unique combination of properties, the metal powders disperseand flow in a paste better than conventional nickel metal powders whichare not small and spherical.

One of the limitations for the application thick film pastes by screenprinting is the difficulty creating lines of narrow width and fine pitch(distance between lines from center to center), and of reducedthickness. The continuing demand for microelectronic components having areduced size has made these limitations critical in component design.One of the obstacles to screen-printing surfaces having these propertiesis that conventional metal powders include an undesirable percentage oflarge particles and also include agglomerates of particles. Either ofthese conditions can produce conductive traces having an uneven widthand an uneven thickness profile. The unpredictable width and thicknessof the conductive traces forces manufacturers to design microelectronicdevices to account for the variations, which can needlessly occupyvaluable space on the surface of the device and waste considerableamounts of paste.

One use for such thick film pastes is in the manufacture of multilayerceramics, sometimes referred to as multichip modules. The packaging ofintegrated circuits (IC's) typically utilizes a chip carrier or moduleto which one or more integrated circuit chips are attached. The modulecan then be joined to a printed circuit board which is placed into adevice, such as a computer. Such modules can advantageously incorporatemultiple wiring layers within the module itself. Multilayer ceramicmodules are typically formed by laminating and sintering a stack ofceramic sheets that have been screened with thick film pastes.

Typically, unfired (green) ceramic sheets are punched with via holes,screened with a thick film metal paste, laminated into a threedimensional structure and sintered in a furnace. The ceramic and metalboth densify simultaneously in the same sintering cycle. Alternatively,the multilayer ceramic can be built sequentially wherein alternatelayers of metallurgy and dielectric are deposited on the substrate andfired.

A schematic illustration of a multichip module is illustrated in FIG.50. The module includes two integrated circuit devices 391 and 392. Anumber of electrically conductive traces, such as traces 393 and 394,are formed on and through the various layers of the device.Interconnection between the two integrated circuit devices 391 and 392or exterior devices is made by the conductive traces and vias, which canterminate, for example, at wire bonding pads such as pad 395 or atconductive pins, such as pin 396. The multichip module can include anynumber of layers, and many such modules include 20 or more such layersfor interconnection. The layers are typically formed from a denseceramic substrate, such as an alumina substrate.

A top view of a multichip module is illustrated in FIG. 51. Electricallyconductive traces 401 and 402 are printed on a ceramic substrate 403 inparallel spaced relation. The conductive traces in such a relation havea design pitch, that is, an average center-to-center spacing between theconductive traces. Manufacturers of such devices desire the linewidthand pitch to be as small as possible to conserve available space on thesurface of the module. However, presently available powders for formingthe conductive traces which contain agglomerates and/or a wide particlesize distribution of particles inhibit the reliable manufacture ofconductive traces having a narrow linewidth and pitch. For example, thelinewidth and pitch for such traces is typically not smaller than about100 μm. There is a demand in the industry to significantly reduce thelinewidth and pitch to significantly lower levels, such as less thanabout 25 μm and even less than about 15 μm.

To achieve such narrow linewidths and fine pitch, thick film pastes willhave to be modified to have improved rheology and more reliablecharacteristics. The thick film pastes incorporating the nickel metalpowders of the present invention can consistently and reliably produce afiner width line and pitch due to the spherical morphology, smallparticle size and narrow particle size distribution of the nickel metalpowders, as well as the unagglomerated state of the powder. Theseproperties will advantageously permit the design of microelectronicdevices with conductive traces having a narrower pitch, and thus reducethe overall size of the devices. Thick film pastes utilizing the nickelmetal powders of the present invention can be used to produce conductivetraces having a significantly reduced linewidth and pitch, such as lessthan about 25 μm and even less than about 15 μm.

The nickel metal powders of the present invention are also advantageousfor use in thick film pastes due to the increased oxidation resistanceof the nickel metal powders. The increased oxidation resistance of thenickel metal powders is due to a number of factors. The highcrystallinity of the powders reduces the volume of grain boundarieswithin the particle and thereby reduces the ability of oxygen to diffusealong the grain boundaries and oxidize the metal. The particles can alsobe coated or otherwise modified to enhance the oxidation resistance, asis discussed above.

As is discussed above, thick film pastes contain a number of organicsused as a vehicle to apply the functional phase to a substrate. Thesepastes are then heated to a low temperature in order to volatilize theseorganics. The organics are preferably volatilized in an oxidizingatmosphere so that the organics are removed, such as in the form ofcarbon dioxide. The temperature at which oxidation of the nickel metalparticles of the present invention begins to occur can be increased, andtherefore, the binder and other organics can be burned out at anincreased temperature and/or under a higher partial pressure of oxygen.The metal alloy powders and metal composite powders of the presentinvention, as discussed above, can also have increased oxidationresistance.

The nickel metal powders and pastes according to the present inventionare particularly useful for the fabrication of multilayer ceramiccapacitors (MLCC's). FIG. 52 illustrates an example of a multilayerceramic capacitor according to the present invention. The MLCC 410includes a plurality of ceramic dielectric layers 412 separated byinternal electrodes 414. The ceramic dielectric layers can be fabricatedfrom a variety of materials such as titanates (e.g., BaTiO₃, Nd₂Ti₂O₇,SrTiO₃, or CaTiO₃), zirconates (e.g. CaZrO₃ or MgZrO₃) or niobaterelaxors (e.g. lead magnesium niobate). Many modifications of thesecompounds are known to those skilled in the art. Terminations 416 and418 for electrical connection, typically fabricated using copper metal,are included at opposing sides of the MLCC and alternating electrodesare connected to each termination. Such devices typically utilize metalssuch as palladium and silver-palladium for the internal electrodes,which are co-fired with the ceramic dielectric. Nickel is asignificantly less expensive alternative to these metals.

Designers of MLCC's prefer the internal electrodes to be as thin aspossible to maximize capacitance, reduce cost and reduce total volume,without sacrificing electrical integrity. Therefore, the powders withinthe paste should disperse well, have a small particle size and containsubstantially no agglomerates or large particles. Powders that do notmeet these criteria force MLCC manufacturers to design the devices withthick internal electrodes to account for the variability. The nickelmetal powders according to the present invention are particularlywell-suited to permit the design of MLCC's with thinner internalelectrodes. Preferably, the average thickness of the internal electrodesis not be greater than about 2 μm and more preferably is not greaterthan about 1.5 μm.

Another problem typically associated with MLCC's fabricated with nickelelectrodes is the sintering of the multilayer structure. Because of thedifferent sintering characteristics of the nickel metal and thedielectric material, many defects can arise in the device such ascracks, delaminations and camber. In order to alleviate some of thesedefects, thick film paste manufacturers incorporate dispersed metaloxide powders in the thick film paste. However, this is not alwayssufficient to eliminate the foregoing problems. The composite metalparticles of the present invention, as is discussed hereinabove, providea unique solution to this problem and can significantly increase theyield of devices. Nickel metal particles that are composite particlescomprising an intimate mixture of the metal phase and a non-metallicsecond phase can advantageously reduce the mismatch in sinteringcharacteristics between the metal layer and the dielectric layer.Preferred non-metallic second phases include the metal oxides and it isparticularly preferred that a material similar to the dielectricmaterial can be used. For example, where the dielectric layer comprisesBaTiO₃, it is preferred that the nickel metal particles include a nickelmetal phase intimately mixed with a BaTiO₃.

An MLCC such as that illustrated in FIG. 52 is typically fabricated byfirst forming a green body, that is, an unsintered structure which isadapted to be sintered to form the MLCC. Thus, the green body includes aplurality of green ceramic sheets with a thick film paste compositiondisposed between alternating sheets. For example, sheets of tape castceramic can be screen-printed with the metal electrode paste and amultilayer structure built by alternating layers. Individual MLCC greenbodies can than be cut from the laminated sheets. The stacked andlaminated structure is then heated to remove organics from the thickfilm paste and ceramic green sheets and sinter and densify the MLCC. AnMLCC including a titanate dielectric is typically sintered at about1100–1300° C. External electrodes can then be applied to complete thedevice.

Another technology where the nickel metal pastes according to thepresent invention provide significant advantages is for flat paneldisplays, such as plasma display panels. Operating under the same basicprinciple as a fluorescent lamp, a plasma display panel consists ofmillions of pixel regions on a transparent substrate that mimicindividual fluorescent tubes. The light emitted by each region iscontrolled to form a video display. Plasma displays can be produced in avery large size, such as 20 to 60 inch diagonal screen size, with a verythin profile, such as less than about 3 inches.

Nickel metal is particularly useful for forming the electrodes for aplasma display panel. A cross-section of a plasma display device asillustrated in FIG. 53. The plasma display comprises two opposed panels502 and 504 in parallel opposed relation. A working gas is disposed andsealed between the two opposing panels 502 and 504. The rear panel 504includes a backing plate 506 on which are printed a plurality ofelectrodes 508 (cathodes) which are printed parallel to one another. Aninsulator 510 covers the electrodes and spacers 512 are utilized toseparate the rear panel 504 from the front panel 502.

The front panel 502 includes a glass face plate 514 which is transparentwhen observed by the viewer. Printed onto the rear surface of the glassface plate 514 are a plurality of electrodes 516 (anodes) in parallelspaced relation. An insulator 518 separates the electrode from thepockets of phosphor powder 520.

A schematic view of the electrode configuration in such a plasma displaypanel is illustrated in FIG. 54. The plasma display includes a frontpanel 502 and a rear panel 504 printed on the front panel are aplurality of electrodes 516 in parallel spaced relation. Printed on therear panel 504 are a plurality of electrodes 508 which intersect thefront panel electrodes 516 thus forming an addressable XY grid ofelectrodes.

Thus, each pixel of phosphor powder can be activated by addressing an XYcoordinate defined by the intersecting electrodes 516 and 508. Plasmadisplay panels can have a large surface area, such as greater than 50diagonal inches, and therefore the uniformity and reliability of theaddressing electrodes is critical to the proper function of the plasmadisplay device.

The nickel powder according to the present invention can advantageouslybe used to form the electrodes, as well as the bus lines, for the plasmadisplay panel. Nickel metal advantageously has a high conductivity andcan be fired in air at the temperatures typically used to form theelectrode pattern. Typically, a nickel paste is printed onto a glasssubstrate and is fired in air at from about 450–600° C. Nickel metal isalso advantageously resistant to corrosion (etching) from the plasmagas. Additives, such as boron or boron compounds, can also be includedin the electrode thick-film paste to enhance the oxidation resistance ofthe nickel metal.

The nickel metal powder of the present invention has a small averageparticle size and a narrow size distribution to provide high resolutionlines which lead to a high pixel density and precision pattern over alarge area. For most flat panel displays, a resolution of at least about25 to 30 μm is desirable. That is, the average line width and spacingshould be no greater than about 30 μm. For higher resolution displays,the resolution should be even higher, such as a resolution of less than20 μm or even less than 10 μm. The nickel metal powders of the presentinvention enable such high resolutions over a large area, whilemaintaining an acceptable yield.

Another class of devices that can advantageously incorporate the nickelmetal powders of the present invention are energy storage devices, suchas batteries. Nickel particles can be used in various battery designs asan active cathode and/or anode material. For example, nickel powders canbe used in nickel-metal hydride batteries and nickel composite powderscan be used in other advanced energy storage devices. The ability tocontrol particle characteristics such as particle size, particle sizedistribution, surrface area, morphology, composition and electrochemicalreactivity in accordance with the present inventions will enhance theperformance of such energy storage devices.

In addition to the foregoing, the powders according to the presentinvention can be used for thermal spraying applications wherein thepowders are agglomerated sprayed onto a substrate to form a thickcoating. The homogeneity and high crystallinity of the particles makethem particularly well-suited for thermal spraying.

EXAMPLES

A number of examples were prepared in accordance with the variousembodiments of the present invention. Examples 1–5 are summarized inTable I.

TABLE I Nickel Metal Examples PRECURSOR REACTION AVERAGE 95% SIZE POWDERSURFACE SAMPLE SOLUTION CARRIER GAS IMPACTOR TEMPERATURE PARTICLE SIZEDISTRIBUTION DENSITY AREA 1 0.5 M 2 lpm N₂ no 1000° C. 1.2 μm 0.4–2.5 μm7.13 g/cc 2.4 m²/g Ni(NO₃)₂ 5 lpm 7% H₂/N₂ (80%) (net 5% H₂) 2 0.5 M 2lpm N₂ yes 1000° C. 0.8 μm 0.4–1.2 μm 7.80 g/cc 1.1 m²/g Ni(NO₃)₂ 5 lpm7% H₂/N₂ (8.5 μm (88%) (net 5% H₂) cutoff) 3 0.5 M 2 lpm N₂ no 700° C.1.8 μm 0.4–3.5 μm — Ni(NO₃)₂ 5 lpm 7% H₂/N₂ (net 5% H₂) 4 0.5 M 5% H₂/N₂no 850° C. 1.5 μm 0.4–3.0 μm — Ni(NO₃)₂ 5 Ni(NO₃)₂ and 2 lpm N₂ no 1000°C. — — — Pd(NO₃)₂ 5 lpm 7% H₂/N₂ (net 5% H₂)For each of Examples 1–5, an aqueous solution of 0.5 M Ni(NO₃)₂.6H₂O wasformed and an ultrasonic transducer generator operating at a frequencyof about 1.6 MHz was utilized to produce an aerosol of liquid dropletsfrom the solution. Nitrogen and hydrogen were used as a carrier gascomposition in the concentrations indicated in Table I. The aerosol wascarried in the carrier gas to a heating zone which consisted of anelongate ceramic tube. The heating zone was heated to the reactiontemperature indicated for each example. For each of Examples 1–5, theresidence time in the heating zone was about 6–11 seconds.

Examples 1 and 2 illustrate the effect of narrowing the droplet sizedistribution of the aerosol, in this case by using an impactor to removelarge droplets from the aerosol prior to entering the heating zone. ForExample 1, no impactor was used to remove large aerosol droplets. Theresulting nickel metal particles had a particle density of about 7.13g/cm³, which is about 80% of the theoretical density for nickel. Thepowder produced according to Example 1 is illustrated in FIG. 55. Theaverage size of the particles was about 1.2 μm and some particlesgreater than about 2.5 μm in size were observed. The particles having asize greater than about 2.5 μm appeared to be hollow shells.

Example 2 was substantially identical to Example 1, except that animpactor was used to remove droplets from the aerosol having anaerodynamic diameter of greater than about 8.5 μm, thus narrowing thesize distribution of the droplets in the aerosol. The powder producedaccording to Example 2 is illustrated in FIG. 56. The nickel particleswere substantially spherical in shape and had a particle density ofabout 7.8 g/cm³, which is 88% of the theoretical density for nickel. Theaverage size of the particles was about 0.8 μm and no particles greaterthan about 2.5 μm in size were observed in the powder and no hollow orbroken shells were observed. The surface area was reduced to about 1.1m²/g.

Thus, the use of an impactor to narrow the size distribution of dropletsby removing large droplets from the aerosol reduced the average particlesize of the nickel metal particles, increased the particle density andnarrowed the size distribution of the powder. In Example 2, at leastabout 95 percent of the particles had a size of 0.4 μm to 1.2 μm. Thus,95 percent of the particles had a size of not greater than 1.5 times theaverage particle size.

Example 3 was essentially identical to Example 1, except that thereaction temperature in the heating zone was about 700° C. The powderproduced according to Example 3 is illustrated in FIG. 57. Many hollowand fragmented particles were observed in the powder indicating that ahigher reaction temperature is desirable to obtain solid, sphericalnickel metal particles. In Example 4, the reaction temperature wasincreased to about 850° C. The powder produced according to Example 4 isillustrated in FIG. 58. The average particle size was reduced to about1.5 μm as compared to about 1.8 μm for Example 3, and the number ofhollow particles and broken shells observed in the powder wassignificantly reduced.

Example 5 illustrates the production of a nickel metal alloy powderaccording to the present invention. An aqueous solution comprisingnickel nitrate and palladium nitrate was utilized to producesubstantially phase pure palladium/nickel alloy particles having a Ni/Pdratio of 70/30. An x-ray diffraction pattern of this powder isillustrated in FIG. 59. The x-ray diffraction pattern illustrates thatthe alloy has substantially no phase segregation of the Ni and Pd andthus formed a well alloyed, homogenous metal.

A further set of examples were prepared to determine the effect ofvarying the reactor temperature above 1000° C. These examples aresummarized in Table II. In each of Example 6–8, an aerosol was generatedusing an ultrasonic transducer at 1.6 MHz. Powder from Examples 6, 7 and8 are illustrated in FIGS. 60, 61 and 62 respectively.

TABLE II Effect of Reactor Temperature Reaction Example PrecursorSolution Carrier Gas Temperature 6 10 w/o Ni 2.1% H₂/N₂ 1000° C. 7 10w/o Ni 2.45% H₂/N₂ 1200° C. 8 10 w/o Ni 2.1% H₂/N₂ 1400° C.It can be seen from the photomicrographs illustrated in FIGS. 60–62 thatas the temperature increased, the particles become smaller in averageparticle size and fewer hollow shells were produced. The powder producedat 1400° C. (FIG. 62), contained few broken shells and the particles arespherical and substantially dense.

An additional set of examples were prepared to examine the affect ofvarying hydrogen concentration at different reaction temperatures. Theseexamples are summarized in Table III.

TABLE III Effect of Carrier Gas Composition Stoichi- Precursor Carrierometric Reaction Observed Example Solution Gas Ratio Temperature Phases9 10 w/o Ni 0.7% H₂ 2.2 1000° C. NiO 10 10 w/o Ni 1.4% H₂ 4.4 1000° C.NiO 11 10 w/o Ni 1.75% H₂ 5.5 1000° C. Ni + NiO 12 10 w/o Ni 2.1% H₂ 6.91000° C. Ni 13 10 w/o Ni 1.4% H₂ 4.2 1200° C. NiO 14 10 w/o Ni 2.1% H₂6.7 1200° C. Ni + NiO 15 10 w/o Ni 2.45% H₂ 8.6 1200° C. Ni 16 10 w/o Ni1.75% H₂ 5.7 1400° C. NiO 17 10 w/o Ni 2.1% H₂ 7.2 1400° C. NiThe carrier gas for Examples 9–17 included nitrogen plus hydrogen, thevolume percent hydrogen as indicated in Table III. The stoichiometricratio indicated in Table III represents the ratio of available hydrogento the amount of hydrogen theoretically required to convert all of thenickel to nickel metal. The amount of hydrogen that was required tocompletely form nickel metal was from 2.1 to 2.45 volume percent of thecarrier gas. This corresponds to from about 6 to about 9 times thetheoretically calculated stoichiometric amount of hydrogen required toreduce all of the NiO to Ni.

As a result of these examples, it was determined that at least about 2.5volume percent hydrogen should be used in the carrier gas at alltemperatures to ensure the complete reduction of NiO to Ni. Thiscorresponds to at least about 9 times the stoichiometric amount ofhydrogen in the system.

Another set of examples were prepared to determine the effect of varyingthe precursor concentration of nickel nitrate in the precursor solution.Powder batches were formed using solutions including 10 weight percent,20 weight percent and 30 weight percent nickel. An impactor was used toremove large droplets from the aerosol, as in Example 2. The results areillustrated in Table IV.

TABLE IV Effect of Precursor Concentration Precursor Reactor CollectionCollection Example Solution Temperature Rate Efficiency 18 10 w/o Ni1200° C. 0.67 g/hr 31% 19 20 w/o Ni 1200° C. 0.55 g/hr 17% 20 30 w/o Ni1200° C. 0.03 g/hr  2%As the solution concentration increased, both the collection rate(powder collected per unit of time) and the collection efficiency(percentage of nickel in solution collected as solid particles)decreased. The collection rate and efficiency were poor for the 30weight percent nickel solution because of difficulty generating anaerosol. The best collection rate and efficiency were for the 10 weightpercent solution. FIGS. 63, 64 and 65 illustrate the powders produced at10 weight percent, 20 weight percent and 30 weight percent solutions,respectively. As the concentration increases, the average particle sizeclearly increases, as expected. Further, as the concentration increases,more ultrafine particles are attached to the surfaces of the largerparticles.

Metal composite particles were also produced in accordance with thepresent invention using the ferroelectric materials barium titanate andneodymium titanate, which are commonly used in MLCC's. The precursor forthe ferroelectrics was prepared by adding either barium or neodymiumnitrate to a water solution containing titanium tetraisopropoxide. Afine precipitate was formed and the addition of nitric acid caused theprecipitate to decompose and form a soluble solution.

Solutions containing 25 weight percent of the ferroelectric precursorwere formed. The solutions were atomized to form an aerosol which wascarried through a heating zone at a temperature of 1200° C. in a carriergas including nitrogen and 2.8 volume percent hydrogen. The resultingpowders were composed of both the phase pure ferroelectric material andnickel metal. A powder comprising about 75 weight percent nickel andabout 25 weight percent barium titanate dispersed therethrough isillustrated in FIG. 66. The metal composite particles are comprised ofan intimate mixture of the metal phase and the dielectric phase.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

1. A method for the production of nickel metal particles, comprising thesteps of: a) generating an aerosol of droplets from a liquid whereinsaid liquid comprises a nickel metal precursor and a reducing agent andwherein said droplets have a droplet size distribution wherein at leastabout 80 weight percent of said droplets have a size of from about 1 μmto about 5 μm; b) moving said droplets in a carrier gas; and c) heatingsaid droplets to remove liquid therefrom and form nickel metal particlescomprising at least about 50 weight percent nickel metal.
 2. A method asrecited in claim 1, wherein said carrier gas comprises hydrogen.
 3. Amethod as recited in claim 1, wherein said carrier gas comprises atleast about 2 volume percent hydrogen.
 4. A method as recited in claim1, wherein said carrier gas comprises hydrogen and an inert gas selectedfrom the group consisting of nitrogen, argon, helium and xenon.
 5. Amethod as recited in claim 1, wherein said heating step comprisescarrying said droplets through a heating zone having a reactiontemperature of not greater than about 1455° C.
 6. A method as recited inclaim 1, wherein said heating step comprises carrying said dropletsthrough a heating zone having a reaction temperature of at least about1200° C.
 7. A method as recited in claim 1, wherein said heating stepcomprises carrying said droplets through a heating zone having atemperature of from about 1200° C. to about 1400° C.
 8. A method asrecited in claim 1, wherein said heating step comprises carrying saiddroplets through a heating zone having a temperature of from about 1200°C. to about 1400° C. and wherein said carrier gas comprises at leastabout 2.5 volume percent hydrogen.
 9. A method as recited in claim 1,wherein said metal particles have a particle density of at least about7.1 g/cc.
 10. A method as recited in claim 1, wherein said metalparticles have a particle density of at least about 8.0 g/cc.
 11. Amethod as recited in claim 1, wherein said droplets have a sizedistribution such that not more than about 20 weight percent of thedroplets in said aerosol are larger than about twice the weight averagedroplet size.
 12. A method as recited in claim 1, wherein said step ofgenerating an aerosol comprises the step of removing a first portion ofdroplets from said aerosol, wherein said removed droplets have anaerodynamic diameter greater than a preselected maximum diameter.
 13. Amethod as recited in claim 1, further comprising the step ofconcentrating said aerosol by removing a second portion of said dropletsfrom said aerosol, wherein said second portion of droplets have anaerodynamic diameter less than a preselected minimum diameter.
 14. Amethod as recited in claim 1, wherein said liquid is a solutioncomprising a nickel metal precursor selected from the group consistingof nickel nitrate, nickel hydroxide, nickel chloride, nickel sulfate andnickel oxalate.
 15. A method as recited in claim 1, wherein said liquidis a solution comprising nickel nitrate.
 16. A method as recited inclaim 1, wherein said liquid is a solution comprising from about 5 toabout 15 weight percent nickel in the form of nickel nitrate.
 17. Amethod as recited in claim 1, wherein said liquid comprises nickelnitrate and hydrazine.
 18. A method as recited in claim 1, wherein saidliquid further comprises a densification aid.
 19. A method as recited inclaim 1, wherein said liquid further comprises urea.
 20. A method asrecited in claim 1, wherein said liquid further comprises a precursor toat least one metal alloying element.
 21. A method as recited in claim 1,further comprising the step of coating an outer surface of said nickelmetal particles.
 22. A method as recited in claim 1, wherein said nickelmetal particles are composite particles comprising a non-metallic phasedispersed throughout said particles.
 23. A method as recited in claim 1,wherein said liquid further comprises a coating precursor and whereinsaid nickel metal particles are coated nickel metal particles.
 24. Amethod as recited in claim 1, wherein said reducing agent is selectedfrom the group consisting of hydrazine and borohydrides.
 25. A method asrecited in claim 1, wherein said reducing agent comprises hydrazine. 26.A method as recited in claim 1, wherein said liquid comprises from about1 to about 15 weight percent of said reducing agent.
 27. A method forthe production of metal composite particles, comprising the steps of: a)forming a liquid solution comprising multiple precursors, including atleast a nickel metal precursor and a non-metallic phase precursor; b)generating an aerosol of droplets from said liquid solution; c) movingsaid droplets in a carrier gas; d) heating said droplets to removeliquid therefrom and form metal composite particles comprising a metalphase derived from said nickel metal precursor and a non-metallic phasederived from said non-metallic phase precursor.
 28. A method as recitedin claim 27, wherein said carrier gas comprises hydrogen.
 29. A methodas recited in claim 27, wherein said heating step comprises passing saiddroplets through a heating zone having a reaction temperature of notgreater than about 1455° C.
 30. A method as recited in claim 27, whereinsaid heating step comprises passing said droplets through a heating zonehaving a reaction temperature of at least about 1200° C.
 31. A method asrecited in claim 27, wherein said heating step comprises passing saiddroplets through a heating zone having a reaction temperature of fromabout 1200° C. to about 1400° C. and wherein said carrier gas comprisesat least about 2.5 volume percent hydrogen.
 32. A method as recited inclaim 27, wherein said metal composite particles have a particle densityof at least about 90 percent of the theoretical density for saidcomposite particles.
 33. A method as recited in claim 27, wherein saidaerosol droplets have an average droplet size of from about 1 μm toabout 5 μm and wherein not more than about 20 weight percent of saiddroplets are larger than about twice said average droplet size.
 34. Amethod as recited in claim 27, wherein said step of generating anaerosol comprises the step of removing a first portion of droplets fromsaid aerosol wherein said droplets in said removed first portion have anaerodynamic diameter greater than a preselected maximum diameter.
 35. Amethod as recited in claim 27, further comprising the step ofconcentrating said aerosol and removing a second portion of saiddroplets from said aerosol, wherein said droplets in said removed secondportion have an aerodynamic diameter less than a preselected minimumdiameter.
 36. A method as recited in claim 27, wherein said nickel metalprecursor is selected from the group consisting of nickel nitrate,nickel hydroxide, nickel chloride, nickel sulfate and nickel oxalate.37. A method as recited in claim 27, wherein said metal precursor isnickel nitrate.
 38. A method as recited in claim 27, wherein said liquidcomprises nickel nitrate and a reducing agent.
 39. A method as recitedin claim 27, wherein said non-metallic phase precursor comprises a metalsalt dissolved in said liquid solution.
 40. A method as recited in claim27, wherein said non-metallic phase precursor comprises particlessuspended in said liquid solution.
 41. A method as recited in claim 27,wherein said non-metallic phase is a metal oxide.
 42. A method asrecited in claim 27, wherein said non-metallic phase is selected fromthe group consisting of titanates, zirconates, silicates, aluminates,tantalates and niobates.
 43. A method as recited in claim 27, whereinsaid metal composite particles comprise nickel metal and not greaterthan about 25 weight percent of a non-metallic phase selected from thegroup consisting of titanates, zirconates and niobates.
 44. A method asrecited in claim 27, wherein said non-metallic phase is selected fromthe group consisting of barium titanate and neodymium titanate.
 45. Amethod as recited in claim 27, further comprising the step of coating anouter surface of said metal composite particles.
 46. A method for theproduction of metal alloy particles, comprising the steps of: a) forminga liquid solution comprising a nickel metal precursors, a second metalprecursor and a densification aid; b) generating an aerosol of dropletsfrom said liquid solution, c) moving said droplets in a carrier gas; andd) heating said droplets to a temperature of from about 1200° C. to1400° C. to remove liquid therefrom and form metal alloy particlescomprising nickel metal and a second metal.
 47. A method as recited inclaim 46, wherein said carrier gas comprises hydrogen.
 48. A method asrecited in claim 46, wherein said heating step comprises passing saiddroplets through a heating zone having a reaction temperature of notgreater than about 1455° C.
 49. A method as recited in claim 46, whereinsaid metal alloy particles have a particle density of at least about 90percent of the theoretical density for said metal alloy particles.
 50. Amethod as recited in claim 46, wherein said aerosol droplets have anaverage droplet size of from about 1 μm to about 5 μm and wherein notmore than about 20 weight percent of said droplets are larger than abouttwice said average droplet size.
 51. A method as recited in claim 46,further comprising the step of removing a first portion of droplets fromsaid aerosol wherein said droplets in said removed first portion have anaerodynamic diameter greater than a preselected maximum diameter.
 52. Amethod as recited in claim 46, further comprising the step of removing asecond portion of said droplets from said aerosol, wherein said dropletsin said removed second portion have an aerodynamic diameter less than apreselected minimum diameter.
 53. A method as recited in claim 46,wherein said nickel metal precursor is selected from the groupconsisting of nickel nitrate, nickel hydroxide, nickel chloride, nickelsulfate and nickel oxalate.
 54. A method as recited in claim 46, whereinsaid nickel metal precursor is nickel nitrate.
 55. A method as recitedin claim 46, wherein said liquid comprises nickel nitrate and hydrazine.56. A method as recited in claim 46, wherein said densification aidcomprises urea.
 57. A method as recited in claim 46, wherein said secondmetal is selected from the group consisting of palladium, gold, copper,tungsten, molybdenum, tin, platinum, iron and cobalt.
 58. A method asrecited in claim 46, wherein said second metal is palladium.
 59. Amethod as recited in claim 46, wherein said metal alloy particlescomprise nickel metal and from about 0.1 to 40 weight percent of saidsecond metal.
 60. A method as recited in claim 46, wherein said metalalloy particles are homogeneously alloyed with substantially no phasesegregation of said nickel metal and said second metal.
 61. A method asrecited in claim 46, further comprising the step of coating an outersurface of said metal alloy particles.
 62. A method for the productionof coated nickel metal particles, comprising the steps of: a) forming aliquid solution comprising a nickel metal precursor; b) generating anaerosol of droplets from said liquid solution; c) moving said dropletsin a carrier gas; d) heating said droplets to remove liquid therefromand form metal particles comprising nickel metal; and e) coating anouter surface of said nickel metal particles.
 63. A method as recited inclaim 62, wherein said coating step comprises contacting said metalparticles with a volatile coating precursor.
 64. A method as recited inclaim 62, wherein said coating step comprises contacting said metalparticles with a volatile coating precursor selected from the groupconsisting of metal chlorides, metal acetates and metal alkoxides.
 65. Amethod as recited in claim 62, wherein said carrier gas compriseshydrogen.
 66. A method as recited in claim 62, wherein said heating stepcomprises passing said droplets through a heating zone having a reactiontemperature of not greater than about 1455° C.
 67. A method as recitedin claim 62, wherein said heating step comprises passing said dropletsthrough a heating zone having a reaction temperature of at least about1200° C.
 68. A method as recited in claim 62, wherein said coated metalparticles have a particle density of at least about 90 percent of thetheoretical density for said metal particles.
 69. A method as recited inclaim 62, wherein said aerosol droplets have an average size of fromabout 1 μm to about 5 μm and wherein not greater than about 20 weightpercent of said droplets are larger than about twice said averagedroplet size.
 70. A method as recited in claim 62, wherein said step ofgenerating an aerosol comprises the step of removing a first portion ofdroplets from said aerosol wherein said droplets in said removed firstportion have an aerodynamic diameter greater than a preselected maximumdiameter.
 71. A method as recited in claim 62, further comprising thestep of removing a second portion of said droplets from said aerosol,wherein said droplets in said removed second portion have an aerodynamicdiameter less than a preselected minimum diameter.
 72. A method asrecited in claim 62, wherein said nickel metal precursor is selectedfrom the group consisting of nickel nitrate, nickel hydroxide, nickelchloride, nickel sulfate and nickel oxalate.
 73. A method as recited inclaim 62, wherein said nickel metal precursor is nickel nitrate.
 74. Amethod as recited in claim 62, wherein said liquid comprises a nickelmetal precursor comprising nickel nitrate and a reducing agentcomprising hydrazine.
 75. A method as recited in claim 62, wherein saidcoating is a metal oxide.
 76. A method as recited in claim 62, whereinsaid coating has an average thickness of not greater than about 100nanometers.
 77. A method as recited in claim 62, wherein said coatinghas an average thickness of not greater than about 50 nanometers.
 78. Amethod as recited in claim 62, wherein said coating comprises a metaloxide selected from the group consisting of SiO₂, Al₂O₃, ZrO₂, B₂O₅,TiO₂, Cu₂O, CuO, V₂O₅, and Bi₂O₃.